I like, and agree with, the vast majority of what you say (especially the protecting yourself aspect), but when it comes down to micromanaging is where I think we fall off the boat. We say they're adults, but we're not willing to treat them as adults. Really, I think the majority of the "my house my rules" mindset is our own lack of emotional ability to let go and only serves to be part of the problem of too many young adults not being able to negotiate their own lives, because they don't know how.
Possibly. See, the thing is we're talking about millions of cases (lot more young folks living with parents these days, due to hard times and whatnot), and each case is individual and different.
Forgive me for resorting, yet again, to personal example. My son is 16 and lives with me full time. He is remarkably sensible, hard working, and responsible for his age. I give him lots of advice and support but relatively few commands... because there is little need to. For the most part I treat him like I would treat any subordinate adult male living under my roof.... because he ACTS like an adult for the most part. He doesn't
need me to steer for him every minute, so I let him steer while I read the map and help him navigate, to coin an analogy.
Now, if he was flunking in school, coming home at 1AM drunk or stoned, or otherwise doing stupid ****, it would be an entirely different scenario. I would HAVE to "take the wheel" for a time to get him back on the road and off the sharp rocks, to continue the analogy.
An adult child living at home as a dependent is a slightly different matter in that they are a legal adult, but exercising some control over a dependent who lives under your roof is not at all unreasonable, especially if they are doing stupid ****. By living under your roof as your dependent, they are to some degree "ceding their personal sovereignty" to their benefactor (their parent and guardian who is paying their way) as a matter of pragmatic reality. Assuming the parent loves their child and wants them to construct for themselves a good life, it is no surprise that they may attempt to exert some control over areas they see as being "problems" in their adult-but-dependent child's life.
Now depending on what individual case or specific example we're talking about, sure you'll have some parents to impose unreasonable conditions or attempt to micromanage to an unreasonable degree.... but the solution to that is "Move Out". Get a job, rent a place, and you can do things your own way under your own roof.... within the bounds of legality and society and your rental contract at least.
OTOH you have young adults living with Daddy and Momma who are doing stupid **** and ****ing up their life and Ma and Pa are trying to get them straight and keep them from driving off a cliff, too.
It just depends on the individual circumstances.
Typically I figure: Reasonable parents + reasonable young adult kids still living at home = reasonable rules.
If either side is UNreasonable then sure things can get crazy.... but again, the solution there is either the young adult obeys the rules for as long as they want the aid and support, or they move out. This is one of the problems that arise with any kind of dependency situation.... the benfactor doing the providing can set almost any conditions he wishes on his continued benevolence, and if the recipient finds those conditions unacceptible his only recourse is to do without the support of the benefactor.