I'm guessing it's because many previous FCP users asked for their money back after paying 200 quid for something they didn't understand.
Whatever the reason, it's a great opportunity for people wanting to test the water.
The best thing to do with a trial like this is to use it to actually produce a finished project. Don't just download it and press a few buttons and do a few online tutorials. Actually shoot some footage and start using FCPX to make something. This will force you to solve problems and really get to grips with the software. We both know that the best way to learn is on the job, so do exactly that.
Sure, choose a simple project or one that doesn't matter if you balls it up, but treat it like a proper project and you'll gain a lot more from the 30 days of free FCPX use.