Technically this is because Linkedin asks to be given access to a user's contacts (phone, email or otherwise) and then displays the contacts as people possible to connect to, regardless of whether the contacts are members already or not. No doubt some manager with shady ethics thought this a very clever way to expand the number of users.
I used to just delete LinkedIn emails but started to hit unsubscribe on each email I got, one month after another. That lasted for 6 months before I flipped and said that this was the only warning I’d give that they’d be taken to court if they ever emailed me again. Not a single email since that point, so an internal blacklist must exist.
You can preemptively add yourself to their "do not contact" list: