I recall a talk by Jeff Bezos regarding starting the 'Prime' business model. He said something like: 'What do you do about people who abuse the all you can eat option?'.

My take is that there will always be a few outliers who take advantage of the 'all you can eat' model. But I imagine most of the time they are just outliers and those people don't affect a business' bottom line that much.

If anything such a model is ethical if it means people can save money. I mean, the business still gets to make money, just a little differently than other businesses.