Statistical studies for types of activities that result in injury or death? Absolutely! "Studies" for these "categorizations"? Not so much, they are just common sense observations (if one were to categorize the different TYPES of hikers, I think these are reasonable).
"Drugs, Alcohol...", etc. Yep, absolutely, but that kind of behavior is not limited to day hikers. Not long ago, there was a firefighter out of Arcadia, California, who by all accounts was an accomplished outdoorsman (having had many outdoor adventures out of state and country). He took an inexperienced friend to the backcountry north of Los Angeles and wound up dead. Although the SAR Director screwed the pooch (had he done it right, the firefighter might still be alive!), an investigation found he was under the influence (drugs). In other words, his OWN fault! But yes, partying, picnics, etc. is not unusual for day hikers. To be fair, there's nothing wrong with day hiking or [most] dayhikers, but the numbers don't lie!
As to the term "Wilderness", the term is VERY broad! It can be an area that is specifically designated so, or simply undeveloped/undevelopable, or it can be the ocean or even SPACE. I consider "frontcountry" that part of the wilderness you can see from "town." Anything over that first ridge, is the beginning of the backcountry.