You are allowed to send ads to your own customers. A bit questionable if that then have to include unsubscribe and doing it when you dont want it. Think if TV makers did the same.
The trick is to slowly add minor amounts of anti-features so users get used to it. Those upset will calm down after a while.
Advertising to customers is one thing but using their own computer they paid for to try and make more money crosses a line. A good dev trys to avoid abusung the power they have over users of their software.