This practice is probably due to the inability of hotels to keep up with changing user habits.

Ten years ago most hotels offered free internet access and really didn't care how you used it. This is because 1) security wasn't as big of an issue to them and 2) not as many people were using the internet. The infrastructure most hotels put in place were akin to the home networks many of us use.

Now most of the traveling world uses the internet on multiple devices, and their simple networks are crumbling. They have to invest in more robust equipment, and they pass the cost on to consumers that use the service.

The mac address process is a vain attempt to stave off the inevitable fact that their networks cannot handle the bandwidth.

My advice is to give your business to hotels that align with your expectations.
For instance, I ask at booking and will not stay in a hotel that has this "nickel and dime" type practice.