While I agree that carrying the child makes you more of a parent than providing DNA (both of which being negligible compared to doing the actual raising), I can understand frustration in this specific context: with surrogacy the people paying for everything are the people providing the DNA, and the person carrying the child is paid for their trouble. If then they have the right to just say they're keeping the child... at the very least they should be required to pay back everyone else.
Which might be the case, of course.
At any rate, it probably doesn't happen that often. It's like how a birth mother has the right to take a child back with adoption (for a period of time that varies from, I think, one month minimum to something like six months or a year), but in effect it very rarely happens.