Also, as Neist pointed out, read the text of the CGS resolution carefully. There are a few caveats in addition to the voluntary nature:
1. It's not the entire school that signs the agreement, it's only the Graduate School division/office/faculty (or whatever entity that is part of the Council of Graduate Schools) of that school that is part of this agreement. This means that programs in things like Education, Public Health, Social Work, Health Science, Business, Law, etc. that are typically outside of the Graduate School's purview are not part of this agreement.
2. The resolution only applies to offers of funding support, not offers of admission. So, in your case, it sounds like they have offered you admission but no funding. They are asking you to apply for funding. To me, this means that they are not subject to this CGS resolution because they aren't offering you money. Of course, once they do offer you money if their program signed onto the agreement, then they should give you until 4/15 to decide if you want to accept the GSA position.
So, I think the first thing you should do is ask whether applying for funding constitutes accepting their offer. It might not be so. I think it's bad practice for programs to require students to accept an offer before they learn about funding, but there's nothing stopping programs from bad practices
