I'm not saying I agree with the bank but they are saying it's part of the severance package. Any employee can just quit if he/she wants to according to the terms of their employment.
But I agree with you, it is highly questionable. What's reasonable? I could say I'm on vacation at any given point in time. If they are asking me to come in I could say "Sure thing, as soon as my doctor says my aids is not contagious". It's just stupid on the part of the bank. They should just offer it with compensation and for the few bucks it would cost them they wouldn't have this bad press and people would be happy to come in. IMHO YMMV