They don't want their trademark turning into a noun (or a verb), just like Velcro doesn't want their trademark to be used to refer to "hook and loop" [1]. It's not unreasonable for them to try to protect their trademark.
Yeah and it's not unreasonable for people to push back and say "I'm going to call them Legos if I want to." Obviously they aren't going to go and sue a mommy blogger because she refers to Legos wrong (much less me saying it on Hacker News), but still.
At least Velcro has a sense of humor about it. They recognize that no one in the real world is going to follow their rules, but they still have to say that they want you to, so the word doesn't get legally genericized.
They probably actually WANT you to use it generically, it helps their brand, since they are only ones allowed to sell it by the word everyone refers to it by. But they can't say that out loud, or it has negative legal repercussions.
1 - https://youtu.be/rRi8LptvFZY