This sounds like it would mean charging Valve money for the privilege of using Valve's own infrastructure every time a player installed a Unity game after a major PC upgrade/reinstall or after uninstalling that MMO they dumped every other game in their library try out.
Steam could probably bake a ban on software that uses installation trackers into their developer/publisher ToS, or ban the collection or transmission of Steam user data related to installations, or something similar.
Tax evasion on top of the other illegal activity can increase the consequences if you're caught. They tend to require warrants before divulging information to other authorities, however any information gathered through legal means can be used against you.
The main benefit of reporting the income is to reduce the likelihood that somebody gets suspicious of the change in buying habits and starts in investigation in the first place.