Maybe it's because both the administrator and the forum moderators have a different view of what a forum is. So they don't see spam and more spam in posts that aren't the pages of a doctoral dissertation, but rather a place where people talk about things just as ordinary people might talk in a plaza or a bar.
That's the thing, a lot of people here don't talk like they do in a plaza or a bar. Or on any other (non) paying forum.
You will seldom see any of those sig banned members writing 1 short sentence post or generally variety in post length, but instead you will most likely see generic 4-5 lines posts over and over and over again, trying to hide meaningless in character count. You peg me as someone (like me) who had a lot of experience on other forums before coming here, and I'm sure you will agree that this isn't how people post elsewhere.
In the end, if forum staff don't think that spam/low quality posts are a problem, why sig ban initiative in the first place?
No, I specifically mean posts like these that I found in 5 minutes:
If we reported these, the forum would be a better place - while still fully respecting the freedom of speech.
Its funny really. If you reported those kind of posts, they won't be deleted (I know because I tried, and even worse than those you shared). But if someone's post history consists mainly of those kind of posts, he will get 1 year sig ban as an attempt to make him wrote fewer of those kind of posts.

edit: I also wonder how many managers would refuse to pay for those 2 posts.