Part of the reason groups might like using MangaDex in the first place is the respect the rules show them in regards to decisions like that, I guess. If a group or two don't want to have their stuff on here, that's their call. That's entirely on a group and host relationship level though, as just an average user it's not exactly like you have no options available to still read their releases if they aren't on here. Between their own sites and the 8 million different aggregators, you can probably find somewhere that works well enough for you even if you'd prefer reading here. I know I'd prefer to read here, but I've got something like three back up options I find acceptable without even counting each group's site.