Why do they do this? If there is no 'legitimate' reason, it's because they're self-centred mindless morons who don't give a damn about anybody else.
It is perfectly permissible to state in your details, when putting something up for auction that, due to previous time-wasters, if payment is not received within 48 hrs. of the auction's end, or contact is not made stating reason for delay in payment, then it will be taken up with EBay and the item will be withdrawn or offered to previous bidder.