It's marketing. They get their name out there for a fraction of the hassle and cost it would take to do it at the same level themselves. Yes, there is PR risk, and that risk might seem foolish now with the benefit of hindsight. But I wouldn't call it "ceding control". You can bet TWR's contract with Trek has all kinds of restrictions and limitations on image and use of the name and branding.It seems strange to me that Trek would cede some control of their brand name to a third party like TWR. People don't know they're not a subsidiary of Trek. They could cause a lot of damage for Trek I would imagine.
And...you can also bet that contract was signed!!