a million years ago when i worked in fleet i asked the same question.
the haulers and lift bed trucks didn't have brakes on their aux axles because *technically* they were only deployed when unloading at the dump/site/etc and they never touched the roadway.
nifty loophole.
also, while working that job a unit came in missing a full set of duals off a trailer bogie. 1) how TF does *that* happen? 2) they never found them and 3) somehow nobody was fired