It’s absolutely unreasonable. Each subscriber has a certain amount of fixed bandwidth available to them, and there are other devices connected to their network. By connecting an unauthorized device, that device now has both the ability to consume bandwidth to which it’s not entitled, and potentially scan the network for vulnerabilities.

It’s basically stealing.

Would you want some device connecting to your network that is not under your control?

And before you answer “well, you should lock it down,” yes, that’s true, but we do not blame the victim if they fail to do so. An unlocked door is not an invitation.