Hmm, so some charter schools basically just give you money and then test every now and then to make sure your children learned something? That sounds remarkably similar to homeschooling (in the more restrictive states), except in this case you're being given money to homeschool. A rather creative way for schools to get generate funding (funding varies according to the number of students enrolled) from people who wouldn't otherwise be interested.
Or to put it another way, the schools get x amount of money for every student, so they give you some fraction of x in return for being allowed to list you as enrolled. So long as they don't dictate what sorts of curriculum you can use (what sort of guidelines do they give for what you can and can't spend the money on?), and tests are infrequent, I don't really see a problem. Though from the taxpayer point of view, schools shouldn't be getting funding for doing basically nothing. If all they're doing is testing, then their funding should be at most a few hundred dollars per student (even counting special needs).
Here's an interesting link on district / charter school funding for various states:
http://www.edexcellence.net/institute/charterfinance/
California, for instance, gets an average of $4,834.97 per charter school student per year. How much of that money are they redistributing to you, and what services are they providing?