Not even give back to the community... just give back to the next person they decide to give their modified version to, paying it forward. They're not under any obligation to give their modifications to everyone.
Good point. That is often one of the most misunderstood aspects of GPL. You have to provide the source code to the user you are selling the product to. It doesn't mean you have to necessarily give it to the whole world or even the original writer of the code.
Not necessarily. The source code might only be of interest to their competitors, not of general interest. They might be at a disadvantage for redistributing some development that they paid for and is giving them an advantage.
Indeed. There's a roaring trade in WordPress themes, even though they're all GPL (or copyright violations). (These frequently license the assets proprietary and call the result an aggregation, but you can still take a theme with cool functions and release your own version with your own assets.)
