What these link indexing services do is take your list of links and put them on thousands of websites they own.
In the hopes that a search crawler visits one of these websites and sees your links. Thats what indexing means, when a search crawler spots your url from another page and adds it to their "index" aka their search database.
To make backlinks out of your links they would need to modify your pages, which they can't. So this is not a way for them to make additional revenue.
Tho there are other monetization methods they do have other than their main indexing business.