Cable monopolies are state sanctioned and state controlled, and thus fall under the Parker immunity doctrine[1]. Its the same doctrine that permits tight state regulation of electric, gas, and other utilities.

[1] https://en.wikipedia.org/wiki/State_action_immunity_doctrine

I would like to correct myself. Cable franchises are state sanctioned and state controlled. Monopolies ("exclusive franchises") were also permitted under the Parker immunity doctrine, but as rayiner pointed out elsewhere in the discussion Congress explicitly prohibited them with various acts in the 90s.

Anti-trust - violating the Sherman Anti-trust Act - is a strictly subjective determination.

Comcast is a huge Democratic Party donor, and their CEO is a golfing buddy of Obama's. There was never a chance the Obama DOJ was going to stop the merger. Democrats traditionally are the harshest when it comes to anti-trust, and you can more often count on Republicans to look the other way, so when you have the Democrats (except Franken!) willing to look the other way too... forget about it. Subjective discretion rules anti-trust enforcement. If the DOJ simply chooses to not pursue it, then that's that.

I'd also argue Comcast is the most convenient scenario for the US Government, from a control position (far easier spying for the NSA and piracy control for Hollywood political donors). Far better to have one large cable entity ala AT&T and Verizon in telephony. They love massive companies, when they're under their boot.

Keep in mind there are plenty of quasi-monopolies that the government is more than happy to encourage or leave in place. From Intel, to Microsoft, to Cisco, to AT&T + Verizon, to Facebook, to Google. The Government has never had a problem per se with monopolies, just monopolies they happen to not like for whatever reasons at the time (often politically convenient reasons).

Telecom is exempt from anti-competition laws since...2004?