(I'm not personally affected by this, but I saw this elsewhere and got a bit puzzled.)
There are cases where a applicant got an offer for a school(A) around now but a school that he thinks is better(B) has waitlisted him and is still pending.
Since A asked the applicant to answer the offer on 15 April, so if he had to keep a school places, he has to reply by then.
But what if B put him out of the waitlist by giving him an offer?
If he took B's offer, he has to withdraw A's offer, and if A refuses to let he withdraw, it has been recorded that some A's would ruin his SEVIS file by saying he is an unreliable person and he couldn't get visa at all.
So what should he choose?