In school, we learned the "proper" way to write, and everyone was supposed to learn the same rules. Those rules worked for both books and letters, but accomplished writers knew when to break them to make a point. Then came the internet, texting, and social media. New methods of writing for new purposes gave way to bending the old rules and instituting new ones. The new system may seem lazy to those using the "proper" rules of capitalization and punctuation, but there's are reasons behind each new convention. Tom Scott explains.
But then we find that the students aren't simply switching codes between casual internet language and formal writing. They just can't do formal writing. We see this in their formal writing in college, which causes writing assignments to be re-designed to avoid formal writing. We avoid gathering evidence that would indicate that the students haven't learned formal writing.
Then the student graduates. S/he sends out resumes, cover letters, and business emails. These are commonly written in casual, sloppy, internet speak not because the student has forgotten to switch codes, but because the student hasn't actually learned formal writing.
There's a great line at the end of the Star Trek episode "Mirror, Mirror." Spock explains that the mirror universe crew members were unable to conceal themselves because, "It was far easier for you as civilized men to behave like barbarians than it was for them as barbarians to behave as civilized men."
This is not to say that people who can't write well are barbarians. But people who have learned the rules of formal writing can break them artfully. They actually can switch codes or registers. But people who haven't learned formal writing can't suddenly begin writing according to the formal rules.
Tom Scott says that people who type in all lower case are actually carefully making a deliberate statement, using lower case to rhetorical effect. I've read far too many student papers to believe this to be true.