The thing with these journals is that they have name recognition that makes people want their research to be published in them. Getting published is good for their career. Combine that with a desire to support open access, and you have a product that you can charge for.
It's doubly good, because it's not like authors are using their own personal funds to pay this fee. It just comes out of their grant. Whenever someone buys something with someone else's money, you know you're going to be ripped off. See also, Wifi at businessey hotels.
Anyway, if the taxpayers are paying for this, we should ask authors to just host the papers on their websites. If you need the $5 for a VPS or whatever, I'll give you the $5 personally.