They cannot do that there's a resolution that says no university can force you so accept or decline an offer of admission before the deadline.
I quote the CGS (counsil of graduate schools) resolution.
"Acceptance of an offer of financial support *(such as a graduate scholarship, fellowship, traineeship, or assistantship) for the
next academic year by a prospective or enrolled graduate student completes an agreement that both student and graduate
school expect to honor. In that context, the conditions affecting such offers and their acceptance must be defined carefully and
understood by all parties.
Students are under no obligation to respond to offers of financial support prior to April 15; earlier deadlines for acceptance of
such offers violate the intent of this Resolution. In those instances in which a student accepts an offer before April 15, and sub-
sequently desires to withdraw that acceptance, the student may submit in writing a resignation of the appointment at any time
through April 15. However, an acceptance given or left in force after April 15 commits the student not to accept another offer
without first obtaining a written release from the institution to which a commitment has been made. Similarly, an offer by an insti-
tution after April 15 is conditional on presentation by the student of the written release from any previously accepted offer. It is
further agreed by the institutions and organizations subscribing to the above Resolution that a copy of this Resolution or a link to
the URL should accompany every scholarship, fellowship, traineeship, and assistantship offer.
The following list includes CGS member institutions that indicated their support of the Resolution.
This Resolution was renewed October 2009."
If you need to you can contact them:
Council of Graduate Schools
One Dupont Circle, NW, 230
Washington, DC 20036
202-223-3791
202-331-7157 (fax)
know your rights that guy can't tell you to make an early decision..
good luck mate