If you are talking about a FT bid cover driver then you are correct that he/she is guaranteed 8 hours of work every day.
If you are talking about a cover driver (we call them casuals here) he/she is only guaranteed 8 hours if they are put to work that day.
Our casuals work just about every day but if they are not scheduled a quick text to a senior driver gets them on road for the day. I don't mind taking a day here or there to make sure they get their hours.
Here, it doesn't matter if you're a bid cover driver or a ft cover driver not by choice. I.e., bumped off route or promoted to full time. All ft drivers get 8.
So you're saying full time drivers can show up for work only to get sent home if no work is available?