They use auto comment that detects new posts or by keywords. Then then spam you with ugly comments. I think of it as a form of vandalism on my feed and will delete and report them as soon as I see them.
The sad part is many Instagram users do not delete spam comments and leave it there. It only encourages such garbage as an innocent party might click or follow the link. You are providing a free advertising medium for spammers and scammers. Worst part; is some even follow back such spammers. The hunger for followers and comments is telling.