Yes, unfortunately, most websites depend on sponsoring, member contributions, and, essentially, on advertisement, etc., but these "legally accepted" entries should stay at their designed spots and not prevent users from reading information or interfere with their questions, answers, chats, etc.
Actually, I have also seen invasive advertisement, spams and scams spreading on all kinds of sites and blogs. Nobody seems to be safe, even with the latest filters.
https://en.wikipedia.org/wiki/Online_advertising
https://en.wikipedia.org/wiki/Spamming
https://www.entrepreneur.com/article/277239
https://en.wikipedia.org/wiki/Internet_fraud
In general, smaller sites might look less interesting for such attempts, but they are still vulnerable.
Actually, talking about Q&A sites reminds me of some other "business practices", like, for instance, late Sodahead - (R.I.P.) ...