They only post in their own communities on their own instance. You will only see their posts if you’re subscribed to those communities or browse All Why would you consider that spam?
There have been times when I've been browsing All and the feed is full of bot spam. It chokes off engagement here and pushes people back to Reddit and the other bot spamming communities, like Hacker News.
If it was in small doses it wouldn't be as bad, but there are times when there are dozens of posts in a row from a handful of communities.
I know that I can subscribe to the communities I like and browse from Home, but that's no good for newcomers to the site, or for people who prefer to use All. You shouldn't have to manually curate your feed to avoid the huge amounts of spam that's designed to take you away from Lemmy.