Simply promoting your product on a forum doesn't count as spam in my books, provided you aren't pretending to be an end user.
Which just so exactly happens to be the thing they are so famous for - they are actually paying people with discounts to post glowing reviews, the more high-profile the forum/website the better. I.e. you don't get (a lot of) coupons/points if you post on your personal blog, places like EEVBlog count for much more - you are supposed to post a link to your "review" and after it is checked you get your discount. Which can be significant - e.g. getting free shipping.
If that isn't encouraging spam, I don't know what is. Then I have seen outright spam from sock-puppet accounts on Reddit too.
So basically your presumed "non-contribution of money to the advertisement industry" helps with spoiling the experience for everyone. When you see a third, fourth, tens, twentieth "review" of PCBWay's boards, the shilling gets really annoying, turning a public forum into an advertising dump. And if you think your boards are going to be cheaper because the vendor spams instead of doing the marketing in more accepted ways - you are being incredibly naive ...
Oh and by the way, PCBWay also buys significant amounts of "normal" ads through those advertisement networks too:
https://www.adbeat.com/free/advertiser/pcbway.com/pcbway.comThat pretty much throws your entire argument out of the window.
P.S.: Ads on a website can be also bought directly, there is no need to go through advertisement industry middlemen for that.