Ads are literal spam. And spam is bad because people are forcing others to look at crap that they never wanted to see in the first place.

If you find these flyers genuinely helpful, and you explicitly gave them your consent to send you the flyers, then by definition it is no longer spam, and thus it is not immoral. But if you did not give your consent, then they are spamming you. How are they supposed to know if you want the flyers if you don't explicitly give your consent?

Maybe if I mailed you my shit, you'd find it useful, maybe not. But if I just mail it to you every week without your consent, it is the same thing as mailing flyers without your consent.