In general, Cryonics organizations have funds and people whose duty it is to revive frozen people. It's not like they're left in a random ditch to be found. Alcor (I don't know about the others) tries to only recruit people to this body who have relatives or loved ones in cryo, and are signed up themselves, therefore having an incentive both to unfreeze existing people and to cause the organization to unfreeze people in the future.
But even were they tossed into a ditch, there are several possible reasons:
1: Research. People from 300 years ago will have a wealth of interesting genetic, bacterial, possibly physiological, psychological, and other differences to study. They'll also have relatively privileged information about archaeological finds and historical manuscripts. I'm not sure how much that part will apply to the future, considering many more records are kept now than were 300 years ago, but the biological parts of the point hold regardless.
Your second question is more a matter of morals but plenty of people do selfish things and yet we don't punish them with death for it.
Sidetrack: If you're cryonicly revived, what are the odds of getting your gut bacteria back?
I think it's past time for another Stupid Questions thread, so here we go.
This thread is for asking any questions that might seem obvious, tangential, silly or what-have-you. Please respect people trying to fix any ignorance they might have, rather than mocking that ignorance.