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.

1. Profit: Reviving people from the past (at least the first time) would make whoever did it famous and respected, which will lead to more grants and other forms of remuneration. You could also put those weird primitives on a reality tv show.

2. Charity: People regularly pay to save stray dogs, cats, etc. from starving to death outside. They not only usually don't receive anything out of this, but in fact contribute to overpopulation of said animals as well as losing money. It would surprise me, assuming the process was not too costly, if there was no one in the next few centuries who felt it their calling to reanimate the frozen altruistically.

3. Experimentation. To perfect a revival procedure, you need to test it out on people. Depending on the availability of future corpsicles, there might be a limited supply, which would bump up people frozen centuries ago into the ranks of the potential scientific revivals.

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.