Because in some situations, this is literally the only means you have to get someone off your network. I'm arguing that it is possible and fairly straightforward to tell why your MAC address got banned. The most obvious is by looking back at your history on the network. You know if you've done something questionable. Another way is to call the helpdesk and ask why you've been banned.

At no point is "I didn't know why" going to be a valid reason for spoofing a MAC address to get around a network ban. If you have the know-how to spoof your MAC address, you have the know-how to understand why it's necessary.

If this happened on my company's network, the next step would be tracking the person down with physical security and letting them know the police would be enforcing the ban next time.