I believe it is because of the 12″ wheels. If I remember correctly the wheels have a max of 55 mph stamped on them. My guess is that they are good to 65, so lawyers said to write 55 on them, then Northern Tools saw that the wheels were good to 55 so their lawyers said to write 45 on it.
This thread discusses that people have used the trailer at 75 mph with no issues: http://teardropforum.com/forums/topic/northern-tool-5×8-aluminium-trailer-mods/
You could also upgrade the hubs/tires if you’re concerned and use the original wheels as spares. I think you’ll still end up a few hundred cheaper than the CLC trailer.
-mike