I'm guessing this is a major part of it. But then again, if a legitimate service is so spammy that people will just "report as spam" rather than unsubscribing, maybe they should change their behavior.
Of course it depends somewhat on frequency etc. as well. I'll still mostly just unsubscribe if there's a link and usually reserve report spam for random crap that doesn't have an automated unsubscribe option.
Pre-Covid, I attended a lot of events and that's probably has led to me getting on a lot of lists. Like it or not, no small part of the event industry is for companies to get leads.
An order form with no pre-selected "join newsletter" option doesn't seem to mitigate this problem. The obvious soloution is to save the customer's email in the mailinglist as unsubscribed, but afaik that's not legal if they request an account deletion.
I find the tabs on my personal Gmail help a lot. Most of the form stuff lands in Promotions or Updates which I glance at now and then. Occasionally there's something I'm actually interested in. I used to keep a separate email for orders and the like but over time I found that just became a black hole that I never looked at.