Charged a $100 'Residential Trouble Call' despite no mention of a charge beforehand, no charge for previous visits, and not fixing my issue

I was charged $100 presumably for the technician visit despite my issue not being solved and this being the third time I've had someone come out to look at the problem. It's not an issue with my setup in any way, but Comcast refuses to do so much as try to figure out what piece of their equipment at what point on the chain is causing my problems. I've given up on getting it fixed at this point as it's an intermittent issue (couple hours per day) that's not on my end, so I've basically been told if I don't get lucky and the issue is happening when the tech shows up then there's nothing they can do. So I'm moving out in a couple months and filed an informal complaint with the FTC. Why am I being charged for a tech visit on a) only the third time I've had someone visit and b) the issue still isn't solved? If I was going to be charged, why wasn't this mentioned to me at any point, especially since the other times techs came out I wasn't charged?