I don't know about the US, but it surely can't be like even greediest ISPs are not having any expenses and just hoarding cash enjoying their monopolies. Even though customers sometimes feel that way.

I believe the following must be true for every ISP on the planet. Hardware crashes and/or dies - power surges, thunderstorms, vandalism, firmware bugs, old age, human errors - all sorts of stuff happen almost daily. It also has to be updated to match with ever-growing bandwidth and latency requirements. The same goes about the software parts as well, although when designed right and working solid it normally doesn't need updates - except for new features - for quite a while. Either way, all that stuff normally requires a reasonably big number of engineers and technicians, always busy either fixing things or building new stuff without introducing any actual innovation (just connecting new customers).

I'm not justifying $50/mo tag (honestly, I have no idea how overpriced it is, although I guess with a proper competition it would be significantly less), just saying that maintaining the real-world (=a world where all sort of stuff that shouldn't be happens on a regular basis) network must cost significant amount.