Previously BostonGlobe.com was doing such thing. They disabled their articles for users in incognito mode and forced users to purchase a subscription on their website. Some websites limit the number of articles people can read without a subscription. I think that’s acceptable as long as websites don’t collect personal information, including but not limited to IP addresses.
If you have same experience about any other website with such practices, please send me an email message (PGP Key).