Only if they get caught. Looking at you VW.

Too bad our supreme court has recently stripped all US agencies of their......agency...

As somebody who once worked at an ISP: they absolutely do that, and it isn't illegal. In fact, ISP's host many of Ookla's speedtest servers. The less infrastructure your test needs to go through, the better the results will be—there's nothing faster than a network that's communicating with itself.