It's unclear exactly why this happens, but managing newsletters in todays "dead internet" filled with bots, scripts and AI fakery is getting increasingly time consuming. For some inexplicable reason bots sign up to receive notifications with emails that bounce back to sender, don't resolve, or simply don't exist.
Adding "double-blind" tests to sign up pages don't work because all that does is require subscribers click a verification link in a email. If the email address is bad from the get-go, verification messages just get bounced back, which doesn't solve the problem of the bad address use in the first place.
There are other things that can be done, but without some form of positive, live-time, check, bad addresses can still be used for subscriptions.
But here's the kicker about newsletter signup spam; there's no immediately obvious "SEO" benefit to newsletter sign up as there's no means through which "SEO" can manifest - no "SEO" link drop, link-jacking, comment or reply spam because newsletters are inherently entirely siloed from these efforts.
So why sign up?
As there's no direct benefit, sign up is not about the newsletter itself, it's reasonable to assume this likely means the underlying send-receive system itself is the goal. Here, mailing systems can be used to test compromised emails, addresses scraped from the internet, or purchased on the "dark web" (they're actually openly available, often though unsolicited direct-to-inbox emails. Ed). Or, and this seems more likely, mailing systems used to send out newsletters can be employed as proxies or DDoS attack vectors bad actors don't need to control, that can directed towards taking down a target, the domain, or business, associated with the subscription email.
Either way, if newsletters result in a lot of bounce-back mail, it's worth unsubscribing those emails to reduce noise (but keep them active in the system so they can't be reused), and to prevent reputational harm to the hosting domain, i.e. katsbits.com.