Bioe makes a good point.

Plugin authors often need to sell something to survive, whether that's a premium version, a subscription, etc. Without that, we wouldn't have a lot of the plugins that make WordPress popular. That means they need a mechanism for letting users know about what they're selling.

At the moment, WordPress is completely neutral on this, and it's up to each plugin author to decide how to advertise and how annoying to be - which the sad result that too many of them have decided to be very annoying. If WordPress had a built-in mechanism for letting you know about upgrades in a non-annoying way, they wouldn't have to abuse the notifications area.

If you want to stop people walking on the grass, give them a footpath.