Quote from ssternlight:
That's total bullshit. Sorry to be so blunt, but that's not actually an offer of employment at all.
When someone offers you a job they want to give you an INCENTIVE to stay not try to tie you up with handcuffs. The exception is when you are offered "golden handcuffs" where you lose a big payout if you don't stick around.
What I would do is indicate that I would be happy to work for the firm but no one knows what life will bring and that this particular term is unacceptable. I'm not even sure it's legal to be honest. If they don't want to hire you without strings attached then thank them and go elsewhere.
Personally, I think they are either testing you to see how stupid you are, don't really want to hire you but can't say no for some reason and invented this silly term, or are so full of BS that you should never work there.
Just my $.02 pence