I don't work at google nor have any details on this, but I would assume that just because they could detect it doesn't mean that it doesn't have a cost and blocking things which don't meet some minimum trust level from Google's perspective would cut a lot of the noise down from naive spamming.
Spam filtering is very hard. Therefore there are some content based rules and some sender based rules. Google is very good at both of those, which means in this context means accuracy and precision based on content and strictness based on sender. Sender based rules make it hard for spammers to send mail pretending to be from a domain, but does make it harder for anyone to send mail. Hosting your own mail server means hosting the authorization architecture to prove you're not a spammer, which makes it harder.
it’s not written clearly, but google does tend to spam filter email from independent domains along with all the obvious spam. they use a legitimate feature as a cudgel against competitors and to lock-in all that juicy personal data flow. it’s one thing to compete with a better spam filter. it’s another to aim it at potential competitors. pretty despicable actually.
