It's the dreaded T-word: telemetry. The same sort of thing people get up in arms about in Windows but JetBrains will get a pass here.
Windows exists to sell ads and Microsoft services, and no one can go anywhere because they're locked into legacy software, and Microsoft constantly abuses that position. JetBrains IDEs exist to sell themselves to businesses as a productivity tool. They benefit from making it actually good because no one is locked in and they have real competition.
Not to mention an OS has more access to my data than an IDE. If JetBrains suddenly decided they'd bundle a feature in IntelliJ where they constantly record my entire screen at all times, I'd be much more wary about their telemetry.
Mandating telemetry for the free version makes a lot of sense. They want to understand how people are using their software so they can improve it. Looking at how someone uses your software before they decide not to buy it seems pretty valuable.
But also I suspect Jetbrains management wants more telemetry period. They've discovered that they don't know how power users work with their software because they've all had telemetry off for years.
Catch JetBrains doing something dodgy enough and there will be a similar shitstorm against them.