Basically it's what tale said in a nutshell. Companies don't want to have to deal with the issues of having an employee being paid by them to create his own thing that he'll sell for a million. I think engineering companies have similar clauses that control IP of their inventors.
But they are not being paid by them to work on their own project, its their free time. Its not even if they use resources from the company that they are working for at the moment, its an automatic measure. It seems too extreme, it will probably kill what little creativity this industry has.
They've been doing it in other creative mediums (as I mentioned, engineering stuff and inventions) as well as including such things as "no competition" clauses for employment contracts every so often.