Why would you NOT want to let higher QoS/CoS tunnels expand?

I'm looking at an implementation where lower QoS/CoS tunnels are allowed to expand if there's unallocated bandwidth, but higher QoS/CoS tunnels are not allowed to. I'm having a hard time thinking of a practical reason for this that you would see in the wild.

The only thing I can come up with is that someone could spoof high priority traffic and cause a DoS attack, but in many cases you shouldn't be accepting the tagging of the incoming traffic, you should be deciding for yourself how you'll tag it.

Are there any other commonly seen reasons someone would want it this way?