Presumably Unity decided they had too many customers and needed to get rid of most of them. Not only is this an insane thing to charge developers for, there are all kinds of concerns like:

• do pirated copies cost devs money?
• couldn't customers organize mass install retaliations to bleed a company dry over any fickle thing?
• how are they tracking installs, and what does that mean for privacy?

This just seems stupid from where I sit.