Are most people working on cars at their home doing it because they're wanting to do modifications most shops won't do, or because they're trying to save the few hundred dollars an hour in labor charges to do some things that take a few minutes of reading and an understanding of lefty-loosey righty-tighty?
A few of my friends and myself tend to do most of the maintenance on our cars and motorcycles by ourselves. If I had to pay for the labor in my car it would have been "totaled" a long time ago. But it still keeps running 20 years and nearly 200kmi with a new water pump, new timing belt, new main belt, new suspension, brake rotor and pad replacements, brake fluid flushes, EGR cleanout, oil pan resealing, power steering hose replacement, transmission fluid flushes, stereo replacement, power window motor replacements, and a few other things.
Most of these parts over the years have been relatively cheap, but getting a shop to do the change would have been expensive as hell. I shopped around to get the struts replaced recently. Most shops quoted me over $1k for the replacement. _Good_ replacement strut/coil/spring/mount assemblies were $60/ea from reputable sources online. So for a couple of hours of my own time, knowing how to unscrew and screw in some bolts properly (torqued to spec), and $240 in parts I had a new suspension. So taking the low end of ~$1k in estimates I got, I pretty much paid myself $380/hr to do the work.
And don't even get me started on motorcycle maintenance. If it were illegal for me to work on my own bike at home I probably wouldn't be able to afford the bike. Parts are stupid cheap but labor costs on motorcycles are astronomical considering its usually easier to work on a bike than a car.
Not everyone working on a car in their driveway are doing something shady or illegal.