You don't know that's why they're ranking. No backlink tool is perfect. There may be links you don't know about that are causing them to rank despite these tactics rather than because of them.
That said, I really don't think there's anything wrong with linking from one domain you own to another domain you own, as long as you're not doing it in a shady fashion. Plenty of legit businesses do this all the time. You want to avoid _excessive_ interlinking of sites, but moderate interlinking is fine.
The juice ultimately has to come from somewhere. Single pages don't generate enough juice by themselves to matter much. Unless you have a vast content farm with tens of thousands of pages (and you manage to get them all indexed), the only way the links from your own sites will be worth much is if those sites have their own inbound links from external sources. If they do, then you control that juice and you can do what you want with it, including pointing it at your own sites.
The more juice you control, the more important you are, the better you rank.
I think where people run into problems is when they try to do this "under the table" and hide a big network of interlinked spammy low-quality sites from Google, and then they get caught. The problem is not that the sites link to each other. The problem is that they're low-quality and spammy. If you own many high-quality sites and some of them link to each other, that's probably not a problem and will probably help you rank better.