Can't the spammers simply strip the subaddress/label after '+' ?
Perhaps the reverse is more effective. Use a subaddress for all your mail and ignore that without a label coming in.
Generally, that's configurable; e.g. in postfix it's set by the optional |recipient_delimiter|. Suppose it's set to ‘-’, and you, John Public, sign up for SuspiciousService using the email address <john+public-suspserv@example.com>. Normally, the mail gets delivered to your 'john+public' mailbox. But if Mr Spammer strips everything from ‘+’, he sends to <john@example.com>. And your system knows that that is not only not a valid local mailbox, but also that it matches a plus-stripped address, and therefore consigns the message and its sender to the fiery pits of hell.
They don't.
It's extra effort for them for nearly zero marginal gain.
I wouldn't say it's nearly-zero gain; by applying a tiny sed expression they obtain a basically unblockable e-mail address.
It's easy to blacklist johndoe+amazon@gmail.com but very few people would be willing / able to blacklist their top-level johndoe@gmail.com. So the spam keeps coming.
Spammers are annoying but the progammers behind them are smart.