It is perfectly reasonable to charge for API access. That's not the problem.
The problem is that Huffman seems to think he can do whatever he wants without consequences. It wasn't "Hey, we're going to start charging for API, here's a reasonable price, and a reasonable time frame, and we want to make sure everyone has a reasonable opportunity to continue providing applications that lots of users and mods use to access and shepherd the site."
It was, "We're going to start charging an exorbitant amount for API access, in an unviably short timeframe. If you have any complaints or disagreements, we're going to provably lie about our interactions with you to make you look bad, oh and we're also going to completely forget that there are people with accessibility needs and basically ignore them except as an afterthought. We're also going to threaten existing mods who don't play along, and replace them with people who do if we feel like it."
That's the problem.