How can kreds be revoked from 3rd party offers?

Two apologies in advance:
1. I am new, I read the sticky, I tried to search, I apologize if this has been posted
2. Sorry for trying to scheme free kreds. Can you blame me?

Okay, so plenty of 3rd party offers give kreds for signing up. For example, Netflix offers a free 1-month trial along with 180 kreds for new users. It does not bill my credit card until one month is done, so I plan to cancel that one in a few days, but keeping the free kreds.

Stamps.com also offers basically the same deal, 180 kreds for a 4week trial. But they say they will revoke your kreds if you cancel.

Can they really do this? How does this work? I plan to spend the kreds right away in Realm of the Mad God. They’ll actually go into my game and undo my purchases just to take them back…? Or do they have the right to bill my credit card somehow then?

Nevermind. A quick Google search shows that this company makes it impossible to cancel, that you will be billed forever. Everyone on Google reports a lot of trouble; I just tried calling their cancellation hotline, even though I haven’t made an account yet, and it’s true… No one is answering…

Companies like Netflix hope that you like their service and convert to a paying customer. Some of the other services are shady and I wouldn’t trust them. Netflix, I’d say, is legit though.