In a sense cartel of carriers has a monopoly. That is even without any conscious wrong doing from their side they act like monopoly forcing prices and terms on manufacturers (breaking the cartel isn't in their interest so even if all of them want to do the best for themselves and don't collude the factual cartel won't be broken).
I was expressing the sentiment and I don't doubt that as the law stands right now it's not formally "antitrust issue". It's however very similar in nature and should be treated as such.
It's close to impossible to introduce a phone without being tied to a carrier. That's exactly what monopoly looks like when dominant player (the one having a monopoly) controls one of the crucial parts of supply chain and can just cut you off if they wish (or if you don't comply).
Usual argument is that it's not a monopoly if you can shop around and choose different provider. Here you really can't without putting yourself in very unfavorable position comparing to players who do comply.
Back to bundling for a bit: it's a practice which is about always bad for a customer. Bonuses, paybacks, plans with phones, loyalty points at gas station. Every one of those reduces competition between providers/sellers by tying customers to them. If about every provider on the market uses the same bundling policy it becomes a monopoly from the point of view of manufacturers of bundled product - they will never be able to compete without bending backwards to meet the requirements of the monopoly owner(s).
That was an idea behind lawsuit vs Microsoft (dominant player bundling their browser making competition (close to) impossible or even "unfair") and that's the exact same situation with carriers or cable providers. Them being a cartel of several instead of monolithic entity shouldn't cloud the issue here.