Although I have heard that several times before and Pandora swears by it, I am not sure that is entirely accurate. It is no doubt an industry standard and the typical limitation, yet Last.fm, Jango, and AccuRadio all have unlimited skips.
In the case of Jango, there is no skip limit because most of the songs are from unsigned artists, and they waive their rights under the DMCA. Last.fm may get around it because it's technically not a US service, even though CBS owns it. Last.fm used to be operated out of the UK, where rules are different. I'm not sure about AccuRadio, but I thought I had hit a skip limit when using them before. I also know some stations calculate an hour differently from others (i.e. some consider an hour from the first time you skip while others consider an hour from when you started your session).
There is no reason to believe any of them have special licensing and no one has complained regarding their unlimited skips. All three are good sized and pretty well known in the industry. In the Launchcast rulings, Launchcast was considered non-infringing despite using standard licensing and allowing their Pro service to utilize unlimited skips. Rhapsody uses standard licensing for their radio feature and also utilize unlimited skips. A lot of other services offer their radio in a similar fashion while using sound exchange licensing.
My understanding was that the pro service of Launchcast paid royalties under the interactive rules. Rhapsody Radio, as a paid service, would probably also pay royalties as an interactive service. Plus, I seem to remember Real Networks negotiated separately with the labels a long time ago since it provides on-demand access to specific songs.
I have read the original document for DMCA and found no mention of a 6 song per hour skip limit. If it is mentioned I am not sure where.
While the DMCA specifies a difference between interactive and non-interactive services, I don't think it specifically says what constitutes an interactive service beyond that it gives the listener control. It was the Court of Appeals that set the limits as to what percentage of a stream a listener can control and still remain "non-interactive." The court basically said a certain percentage of the stream had to be songs for which the user never expressed a preference. The 6 skips an hour is the way some streaming services can insure that percentage is hit. So, yes, it looks like there could be other ways to guarantee the user doesn't exceed that percentage, though I have a hard time coming up with them if the skips are unlimited!