These "bots" are spam, pure and simple. By forcing users to visibly name the "service" they want to use in public discussion threads, and the service visibly answering on the same thread, they are basically forcing the users themselves to advertise the service. But it's still spam. You get tens of mentions of the service on each reddit thread.

It would be terribly simple and probably even more userfriendly to just privately message the bot in order to activate its action, and/or much more tech-friendly to have a browser extension to do the same action. I'm quite sure youtube-dl already works with more types of "Reddit videos" than "SaveVideo" ever will, and youtube-dl couldn't care less if Reddit approves of them or not.

But, of course, thanks to the apparent power of advertising, any such service that does NOT spam everyone like "SaveVideo" and the rest is condemned to obscurity.

Net result is tens of annoying SaveVideo (and their even more spammy competitors) messages in every popular reddit thread. How do you call it when someone posts several messages in every single popular thread with the name of their service?

I really don't know why the websites (Twitter, Reddit, etc.) put up with those. Yes, Twitter has Threadunrollers and the like which is basically the same thing. Personally I consider these "business models" to be one of these plagues on the WWW that must be eradicated with prejudice. The other one being websites that just grab files that are otherwise publicly available and put them behind a loginwall/paywall (e.g. Pinterest, Scribd, Manualslib, etc.), then SEO the hell out of it so that the original file becomes much harder to find.