I don't know how much work has gone into making these content blocker apps, but I bet it has taken some time - time that in any "normal" business would be charged for, and I think mobile apps creators should charge more for their apps too. In my experience, there are two types of mobile apps consumers. Those that won't pay for an app, whether it costs $1, $10 or $20, and those that will, and I think it would be healthier for the ecosystem to cater at least as much to the latter group as the former, but I don't think that's currently the case.
It's a race to the bottom, folks.