So there are 730 hours in a month. You're insured for all of them, but you might be driving for only 200 of them, and a portion of that won't be "on a trip". What percentage of the monthly commercial drivers insurance cost are they counting as expenses?

Same thing with maintenance. Yeah, that oil change may have cost $40... but none of it was incurred while on a trip.

ceejayoz didn't say slippery slope - they mean slippery as a synonym for "tricky" or "furtive"

For example, "$21 per hour while on a trip" could mean that, if a driver takes a fare 30 minutes out of town then returns empty, they are only paid $10.50 for the hour's work because returning empty was not "on a trip"