This argument doesn't work. Just because you already have heavy regulation, doesn't justify having more regulation.

Well, it doesn't by itself justify more regulation, but it makes additional regulation less burdensome. If trains were not regulated and planes were, it might be reasonable to add regulation of bathrooms to plane regulations, but not to introduce regulation to trains so we could regulate bathrooms.

Also, many libertarians would say that the solution should be to simply remove much of the heavy regulation of air travel.

Fair enough.