If it is a two-wheel bike trailer - like the ones normally used for hauling children - then the issue is probably more one of stability during turns as this type can flip over if turning too fast, or if one wheel hits a large bump unexpectedly. If it is a single wheel trailer like BOB trailers, then there is really no reason to limit your speed. Perhaps the warning is a CYA type statement so if you are hauling 400 lbs of lumber and crash into a tree they can say 'we warned you'.