TL;DR version: if disabled people have a harder time getting good jobs, it's discriminatory toward them to require good job experience for entrance to your graduate program.
I think your immediate reaction is likely to be "no," but let's explore this a little. I think the logic behind most programs is something like this: we think professional experience helps to flesh out the content of our program, and so we generally require it. If you don't have the work experience, we recommend that you take some time, get that experience, and apply again some other time. Assuming programs don't mean to be discriminatory, there's an assumption at work here that getting professional experience is not unduly onerous for groups on race/class/gender/etc dividing lines.
My point here is that disabled people have a tougher time getting jobs at every step of the way, which makes them less qualified for professional jobs, and this in turn makes them less qualified for programs that require professional experience. This should be pretty intuitive: if you're disabled, you generally have trouble with those first jobs you can get, which usually require some basic form of physical involvement (at least in my area). In my case, I have a mobility disability—walking, standing, and so on. So it's pretty easy to see how the wide range of jobs that involve standing behind a counter or doing physical work are off-limits to me. But for a college-age person, that's a large fraction of the available jobs. Furthermore, in my area, desk jobs are rarely entry-level jobs. (I should also point out that, in my area, internships are not much of a thing. I don't know anybody who had an internship in undergrad.)
I'm saying that if you don't have those starter jobs, it's difficult to secure those second jobs, and it snowballs from there. In particular, it can be difficult for disabled people to land those kinds of early-career desk jobs. I don't want to speculate too much, but I think managers are more comfortable hiring early-career candidates with lots of work experience, and it's easier to get that experience if you meet certain physical standards. And so on: the people who get those early-career jobs get those professional jobs, and this would tend to exclude disabled people. This tendency toward exclusion would weaken disabled people's position with regard to admission to MPA programs.
So, to look at some statistics, we would expect to find something like this: disabled people are employed at a low rate, disabled people are employed in non-professional jobs, and so on. Here's what I could find:
In 2014, 17.1 percent of persons with a disability were employed, the U.S. Bureau of Labor Statistics reported today. In contrast, the employment-population ratio for those without a disability was 64.6 percent. http://www.bls.gov/news.release/disabl.nr0.htm
In 2014, 33 percent of workers with a disability were employed part time, compared with 18 percent for those with no disability. (ibid)
Persons with a disability are less likely to have completed a bachelor's degree or higher than those with no disability. Among both groups, those who have completed higher levels of education were more likely to be employed than those with less education. However, at all levels of education, persons with a disability were much less likely to be employed than were their counterparts with no disability. (ibid)
Workers with a disability were more likely than those with no disability to work in production, transportation, and material moving occupations (15 percent compared with 12 percent). Workers with a disability were less likely to work in management, professional, and related occupations than those without a disability (31 percent compared with 39 percent). (ibid)
In 2014, 15 percent of workers with a disability were employed in federal, state, and local government, about the same percentage as those with no disability (14 percent). (ibid)
The unemployment rate for persons with a disability was 12.5 percent in 2014, about twice the figure of 5.9 percent for those with no disability. (ibid)
Across all age groups, however, persons with a disability were more likely to be out of the labor force than those with no disability. (ibid)
At the master’s level, 8% of all students with disabilities and 7% of all students without disabilities were enrolled in social and behavioral sciences.(From 2011) http://cgsnet.org/data-sources-graduate-students-disabilities
So, given that stuff, we can probably conclude that disabled people do worse in the economy than non-disabled people. They're less likely to be found in professional environments than "normal" folks, but perhaps not by a huge amount. Still, there is no accounting for how those people got there. My claim to start was that disabled people would have a harder time finding a professional job for experience, and I think this data seems to hint at that. While the stat on master's programs would seem to have disabled people actually overrepresented in social science master's programs, it's important to keep in mind that very many of those programs do not require work experience of any kind. In fact, most fields do not require work experience at all, and so we can't draw much of a conclusion from that. Still, I think there's an amount of support to be drawn for my basic claim from what we've got here.
Thoughts?