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.