The American thousands of pounds thing is even more weird than that: I buy gravel from the gravel pit. They weigh my truck and dump trailer in lbs and then charge me per ton. Everyone talks about tons: e.g. I'd call a dump truck operator and ask him to deliver 18 tons of rock, not 36,000lbs. But the weighbridge and road signs and regulations about vehicle and axle weight are always in lbs. If you only read things and never talked to other people you'd never know tons existed.
I wonder if using pounds exclusively stems from a time when the word ton was potentially ambiguous. In both British Imperial and US Customary one ton means 20 hundredweight (20 cwt) but the hundredweight differs between the two countries, 112 lb in Imperial, 100 lb in US Customary resulting in the US Customary ton being the short ton of 2 000 lb and the Imperial ton being the long ton of 2 240 lb.
Using only lb avoids confusion.
A lot of the older "no vehicles over X" signage is denominated in tons.