[notahacker]

Cabotage restrictions really don't have anything to do with why Ryanair (or easyJet or Wizz) don't operate in the US though; it has no overlap whatsoever with the market they operate in, they don't do connecting flights and if the economics favoured the low cost model expanding to many more routes in the US then Southwest or US-based startups might have made more of a go of it in the last two or three decades (the US doesn't exactly suffer from lack of people with the knowhow and access to capital to start airline businesses if they think some routes are under-served). It's also possible for sufficiently motivated foreign entrepreneurs to circumvent with subsidiaries and joint ventures: the AirAsia operations are a good example of this.

Cabotage might kill off a handful of potentially viable routes between the largest cities for relatively unusual multi-stage flights, but it's not really the reason why most domestic routes end up being local monopolies or duopolies. Emirates might have an interest flying an NYC-San Francisco leg, but it's difficult to imagine a foreign competitor being interested in a Denver-Houston route, never mind Denver-Wichita or Denver-Durango