What federal regulations are you talking about? Most cities seem to at least have normal buses to their airports — all those service employees have to get to work somehow!
Seattle now has light rail directly to the airport, and Portland has for a while. In the DC area there's National just across the river with a normal subway stop and normal public buses to BWI and Dulles in the suburbs (that stop at subway stations).
I was referring to direct subway interconnects to airports being very difficult to fund, but now I'm uncertain. I was always told growing up (80's - 90's) that the reason that reason for the ridiculous off-site airport terminals for subway systems was because of federal regulation prohibited the use of certain funds for air travel. I was always amazed to see how smoothly transport to airports worked internationally, and depressed by the difficulty in the US: Boston, New York, SF.
Since then, all of the cities have changed, and now have better direct connections from their subway systems. I presumed this was a legal change, but now I'm unsure. I can't find anything directly confirming this. Here's one of the closest I can find: http://sf.streetsblog.org/2010/06/22/bart-moves-ahead-with-o...
In this case, it was determined that BART did not comply with the Civil Rights Title VI regulations regarding equity of spending for public funds, and thus was denied the use of federal funding for an Oakland subway extension. I believe this was the basis for the earlier prohibitions, based on the (reasonable?) presumption that the poor do not benefit proportionally from having easy airport access and the rich don't need the subsidy.
There's a lot of big cities that have horrible mass transit period let alone airport service. How about all of Florida, for one. TPA is my home airport and it's a beautiful airport but there's no significant mass transit service. Likewise for Orlando and Miami.
Seattle now has light rail directly to the airport, and Portland has for a while. In the DC area there's National just across the river with a normal subway stop and normal public buses to BWI and Dulles in the suburbs (that stop at subway stations).