A bit of chatter here about "it eventually catches up to people anyway" that I think misses the point. You want people to learn when the stakes are low. This is better for them, and it's better for all of us. School should gradually go from low stakes to higher stakes, not be a low-stakes zone the whole time that then tosses people into the real world. Otherwise you're presented with terrible choices like expelling a…
When are the stakes low vs. high when it comes to academic plagiarism? I would think that the stakes are always quite low, at least until you get to post-graduate academic work where plagiarism can have legal or serious ethical (e.g. medical research) problems. You seem to be claiming that at some point the stakes become "truly" high, and thus we ought to artificially punish plagiarism so that people learn that there…
The high-ness of the stakes at the college level seems to depend on if you have scholarships or similar - I've seen those be pulled, but haven't seen people I've known personally be actually expelled. So if you're fully dependent on them, it's bad, otherwise, it's no big deal. After college it depends on the type of job - some bigco or government orgs seem perfectly happy to carry a bunch of dead weight around.
Pre-college is mostly a super-low-stakes area for kids these days, but complicated by something 'tcfunk points out in a sibling comment, where authority figures lie about that - claiming everything is dire - but the kids also know their teachers/administrators are full of shit. My friends and I were very good at finding out which rules we could break with impunity - having too many rules, if anything, watered down the seriousness of the important ones.
So where you see problems are e.g. kids who could get away with anything suddenly running afoul of drug laws, or losing their scholarship over rules that didn't have teeth in high school, etc. Of course, the better off you are, the more easily you can weather that stuff - but to me, the root is that we're very opaque with our kids about what you'll get into real trouble over or not, and it changes in a very lumpy manner.