You seem to be confused regarding who is engaging in the DoS attack.
The script poisons bittorrent traffic in order to *stop* the DoS attack that is *already* in progress, caused by the *torrenter*.
This is not the equivalent of yelling at kids riding their bikes. It's the equivalent of having a bunch of kids forcefully and willfully blocking the driveway of you and every other person's house on your street because they want to play road hockey.
In real life, you'd be calling the police and possibly pressing charges. But that's a lot harder to do with network traffic, hence people going vigilante like this. This is an inevitable consequence when more reasonable measures are not available.
Your strawman arguments are ridiculous. Why the hell would the owner of a coffee shop be in the back room downloading CentOS ISOs? That's horse shit and you know it.
The whole point of a business providing public wifi, is to entice people to come in and use their other services. If I go in there with the expectation that I can peruse slashdot while enjoying a cappuccino, and I can't, then that will directly impact my decision to go there again.
I challenge you to find one single shop owner that wouldn't happily invite measures such as these, so that they are able to provide service to their customers.