The 45 mph thing is just a CYA, like all others have advised.
ALL Harbor Freight trailers come with that warning in their instruction manuals. I have a Harbor Freight that I pull behind the motorcycle all day "just a tad" over 45 (about 25-35 "tads" over) on the highway with never a problem.
As far as needing 12" tires and a longer tongue to be able to go over 45 mph... again, there are 8" tire-ed trailers out there that run thousands of miles at 60-80 mph without a problem. The greater tongue length and larger tires make it tow better, but are not absolutely necessary.
12" or 8", either way, what I did, and what everyone should do with these HF trailers is to get the grease off the bearings and repack with a better quality grease. I went a step further and changed out the bearings to Timken's in the hopes of ending up with a higher quality bearing than what I am sure were originally on the HF. I figured it's cheap insurance with the important moving parts.
On my website, there is a page about the HF trailer and towing with it, including the Timken part #:
http://www.valkyrieguy.com/mctrailer2.htmRemember, I am not an expert... YMMV