Safe? Safe for you, certainly. Safe for the life of the vehicle may be another question, but even that is iffy.
The tow speed limits are intended to reduce the heat in the transmission caused by towing. Without the engine running to circulate fluids and cool the tranny, some vehicles may experience greater than normal wear and tear. Enough to be harmful? Probably not - the engineers who comne up wioth these things are pretty conservative and are trying to keep their warranty costs to a minimum. Until recently we towed Chevy Trackers all over North America (two different ones) for a total of over 50,000 miles. We always ignored the 55 mph tow speed limit and towed at whatever speed the highway safely handled, but we did run the engine every 200 miles to circulate fluids, as recommended.
There are plenty of vehicles with no limit, if you would rather avoid the issue. The entire Jeep line, the Honda CRV, and most of the GMC and Chevy SUVs and pick-ups. The new crossover SUVs from GM and Ford share a new 6 speed automatic transmission that is towable 4-down - check out the GMC Acadia, Saturn Outlook, Ford Edge and Lincoln MKX