Where in the FMCSA regulations do it say about drivers being stopped without reason?

No regulation, you drive a CMV which is subject to search at anytime on any public road.

 

They don't need a reason to stop you for a inspection.

 

Here a short explanation, or go to CVSA inspection criteria.

 

Your boss, if he's been in the trucking business any time at all, should already know this.
He's messing with you, trying to see if you had done something to cause you being pulled over.

 

Many years ago there was language in part 350 that got into detail about MCSAP and how to conduct these stops. All this got changed some years ago that language is no longer in that regulation. What has happened over the years is these stops are allowed more and more because Trucking is a heavily regulated industry. I don't like it, but it is what it is.

 

The exact regulation you are looking for is § 396.9 Inspection of motor vehicles and intermodal equipment in operation.located at https://www.ecfr.gov/current/title-49/subtitle-B/chapter-III/subchapter-B/part-396#396.9

396.9 Inspection of motor vehicles and intermodal equipment in operation.
(a) Personnel authorized to perform inspections. Every special agent of the FMCSA (as defined in appendix B to this subchapter) is authorized to enter upon and perform inspections of a motor carrier's vehicles in operation and intermodal equipment in operation.

There was also a federal court case trying to fight it, but failed. If you really are interested you can look up US vs Ford. It's at FindLaw's United States Fifth Circuit case and opinions. and has tons of cites and other case law on why it's legal for officers to pull over a CMV.

 

A roadside inspection is a valid reason to pull a driver or commercial vehicle over. You can be pulled over randomly, for cause/suspicion, or no reason.