A closer analogy would be that its like if you go through a Burger King drive-through they also give you gas to bring your tank back up to what it was when you started on your trip to Burger King.
You said that "The job of the ISP is to provide network connectivity, not decide how I use it".
My point is that if you have paid for a fixed amount of data per month, and you are going to use under that amount when you decide entirely on your own how to use it, then whether or not the ISP also gives you some free data with usage restrictions doesn't change that.
If your ISP does not have any exemptions and then adds some and you miss the announcement you would never even notice, except maybe if you track your usage throughout the month to make sure you aren't going to go over and you notice they are reporting lower usage than you expect.
Yes, it is putting a thumb on the scale, but I don't see how this is really different than any of the other ways companies often give free access to third party products to try to make the company more attractive to customers.
That may be anticompetitive, but if so it should be dealt with under antitrust. Network neutrality should just cover things that restrict what you can do with the network.