The idea behind it is two-fold........
#1.) Vehicles towing need A LOT more room to stop!! (Trailer brakes help, but are not the save-all)
#2.) Typically speaking while the *vehicle* may be rated for more than 55mph, the trailers often are not..........especially in the axle, hub (bearings) & tires.
That said, I really don't see anything wrong with them staying up with the flow of traffic in the slow lanes. Really driving 55mph creates nothing more than a hindrance. On today's freeways, I think that speed limits should probably 65mph for towing and 75mph for everybody else. Last night I drove I-5 from Mission Viejo to Anaheim and I was going with the flow of traffic at about 80mph.
For the record, I used to have a Sea Doo with Zieman's smallest trailer that had 13" wheels. I *could* do 75 but got sever shake.
3 years ago, I got a ticket on I-40 going to the World Finals and towing 2 skis on a Zieman double w/ 15" wheels.(big difference from my first trailer) I was doing 72mph passing a line of about 5 big rigs in a row. I let the officer know that while I knew & respected the law, I thought it was pretty sad pickin' on me for *that*(he knew exactly why I was doing 72) when I was passed in Barstow by similar rigs doin' about 85mph!
My .02!