As you can see in the list many of them have had to shut down because they couldn't make a go of it. Some seem to operate sporadically since they have limited amounts to give out, it seems, like Bitcrate now that it is under new ownership.  Others use it as a way to promote their site and so it is no doubt seen as an advertising expense. 

For example Coinbase is right now giving out the most of any of them, 0.10 BTC where most only give 0.0001 BTC or so, but they seem very happy with the interest and attention their give-away is generating for their site.