Having worked as a rafting guide a few years back, this seems to be standard practice in outdoor type companies. People don't turn up, you don't work, you don't get paid.
Secondly, its not unreasonable at all. Sure the trip you may have done on vacation was an easy float down a class 3, but that's the equivalent of hiking a small hill an hour from your home. There are plenty of rafting expeditions that probably took way more planning and organisation and unknowns than some rich kid paying a million bucks for some company to hold their hand all the way up Everst.