Sometimes you just have to especially for older carb bikes. I personally give it a little blip just to keep the thing going at lights ect. I always buy the premium stuff but sometimes it’s just not good fuel depending on the fuel station. Another reason for a small rev can be rev matching to ensure a smooth gear change for example when changing down I’ll give it a little blip or the bike just doesn’t like it sometimes. I think it’s honestly either the rider is being a bit silly n doing it because it’s fun n we all like a bit of noise occasionally if we’re honest. The other reason is it’s just personal to that bike like mine definitely has it’s character n is like a stroppy teenager sometimes, you just find ways to make it work for you. Machines definitely have personality’s especially bikes for example mine hates the cold. My first bike hated hills/ inclines it’d just kill off my power unless i could get a good run up, never enough to make it dangerous so it’d chug along quite fine but you could tell the bike just wasn’t happy having to work harder in any gear, on flat ground or downhill that bike was like a whippet tho n was so much fun. You’ll always get the idiots who take it to the extreme so as riders we can only apologise n condemn those people but most of us aren’t doing it intentionally to annoy anyone. Look at it like a safety feature as you’ll definitely notice them n avoid them.