My pleasure! Let's go by parts:
most of all people on the planet listen to music in some capacity
That's right. If their ears work, people listen to music.
whether it's great or small doesn't matter. It's like putting "watching TV" or "shopping for groceries
I get you. You're talking about the casual listeners. These are the people who listen to whatever music is playing on the radio at the moment or whatever is popular right now. When I'm talking to someone and they say "oh, I like all kinds of music" and they don't go in detail (genres, bands, etc), I identify that person as a casual listener. No problem with that, this is how most people start.
After that, you get into a single genre. I live in a place where those who care about music are crazy about rock and heavy metal. They listen to the most famous bands of their preferred subgenres. You could say this is the second level of the casual listener, along with the die-hard fan of a single band.
All are proper activities in which not the absolute majority of the developed world is involved. Hobbies are activities that distinguish you, that make you you, and I can't grasp how doing that thing that 90% of the developed world population does makes you any different.
Hmm... I think you should take a look at this list I shared with yellowoftops.
This is the difference between casual listeners and people who love listening to music. I started that list around 2 years ago. I usually go through the entire artist's discography, listening to the albums front to back. It used to be a single genre for the first 100-150 albums, but then I felt an urge to diversify a bit. Now, I listen to around 2-3 albums a day: if you look at the end of the list, you'll see that right now I'm oscillating between metal and disco.
About doing something that 90% of the developed world population does... well, we get a lot of great music with each passing day, and one simply cannot listen to ALL the music out there. I tried finding some bands in common with the people I used to talk about music, but it wasn't of any use: all the bands I knew were unknown to them, and they didn't know any of the bands I liked. After the 300 albums mark, I just stopped name-dropping the bands I liked, it's just so many! As time went by, I noticed my taste got broader and more specialized.
In closing, it classifies as a hobby for me because I get a craving for new music whenever I stop looking for new stuff. And there are the "oh, there's nothing interesting out there" cycles every 3 months.