(I don't think spam flags are personalized, so even to me future emails wouldn't go to spam. I could set up a filter, though.)
EDIT: To clarify: in Germany you can sue companies for spam and making the opt-out unnecessarily difficult. Of course the damages are so trivial you'll likely overpay but there are enough bored lawyers that these things tend to happen every now and then.
I looked up the federal law at one point and private individuals had no rights beyond reporting to the FTC. ISPs may have standing if they can show harm.
See https://www.law.cornell.edu/wex/inbox/can-spam_standing and http://www.dwt.com/advisories/9th_Circuit_Deals_Blow_to_Prof...
I see now that the majority of this document could have expired. Not sure where to find these "notes" that the document talks about. It also says federal law would preempt. So yeah... talk to a lawyer I guess!
As I'd just noted in another comment, many senders use a mail service provider, and a single bad actor can have very bad knock-on consequences, even for mail which doesn't transit the MSP (SPF records listing MSP IP space).
For many major brands / companies, unsubscribes work reasonably well. I've seen some success with this, don't offer email addresses generally, and flag the remainder as spam, with blackhole rules in my various filtering tools.
The larger problem with email is I simply don't trust it any more.
Signed up for pickydomains a while back, they send me emails every time there's a new listing. I decide I don't want to get the email anymore. Click the unsubscribe link. They want me to sign in. I sign in, and am unable to do anything without agreeing to their changed Terms of Service.
In other words, I have no way to get them to stop emailing me without agreeing to new terms.
Similarly, I once signed up for a trial of IBD newspaper. I still get emails from them occasionally (although those at least go to spam), and the unsubscribe link requires a sign in, which I don't remember. I could probably reset my password if I cared enough, though.
(These are just the two I remember, I'm sure there's been others that were similar. I had an issue with PayPal spam at one point as well but it was more complicated.)
As far as I can tell, both of those are in violation of the CANSPAM act. Companies are required to offer an opt out link with no additional requirements to opt out. But there's no consumer right of action, so the only one who can enforce it is the government, and they focus on worse spammers, so real businesses that are just aggressive mostly get off free.