The issue is that SPAM is defined by the recipient, not the sender. Google sends out SPAM, and people marking those emails as SPAM trained Gmail to filter out Google's SPAM.

Some examples of spam from Google:

Years ago I unsubscribed to emails from YouTube, but they just kept coming. I kept hitting that SPAM button.

I get a lot of pointless informational emails from Google that I can't unsubscribe from. I hit SPAM on all of them.

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The point to remember is that there's no magic rule to follow that makes bulk / automated email "not spam." If enough people think it's SPAM, the filters will learn to filter it out.