I can't speak to the 1.5 as that may very well be an engine limitation but for the BSBL and Mav Tremor it is the twin-clutch that's the limiting factory (engineering limitation) as it wasn't designed with towing in mind. I don't have the source handy but it is legit. That's why for the Mav...
The lower tow capacity for the BS BL is the Advanced AWD system adapted from the Focus RS. It is apparently an engineering/design limitation of the drivetrain. That is why the Maverick Tremor has the same pathetic towing as the BSBL while the non-Tremors can do 4k Tow with the regular AWD...