I've heard of bosozoku, yankee and highway racer style applied to them, maybe zokusha is slang for bosozoku or something.
They're just a group of enthusiasts who style 1970's and 1980's cars kinda in the theme of the then race cars, and then some. They're pretty much the Torana and Gemini drivers of Japan. Anyways, they've been around a lot longer than the drifters... and the reason they stretch tyres is:
1. its a lot cheaper to stretch a narrow tyre onto a wider rim, than buying a wider tyre.
2. its part of the "look"... so even though they may own a $50,000 KPGC10, they'll still stretch the tyre because its part of the tradition now.
3. They don't give a rats ass about performance. Its the stigma they care about.
Now fast forward to Australian "drift look" cars. I use this term because half the muppets running around these days with BN Sports kits and vertex kits and "omg dishhhh" wheels with stretched tyres, would get their panties all up in a bunch when you ask them to drift their car for real. I mean... who'd want to gutter bash their $3000 Professor SP1's right? And no fishies down a wide road or getting your arse end out around a curb in your neighbourhood doesn't cut the mustard either.
So onto the real reason for stretching your tyres if you are a drifter:
1. 99% of real drifters aren't sponsored, are poor and can barely afford to keep their cars on the road. Wide tyres are out of the question. Find tyres that your local tyre shop or service centre is throwing away (out the back, ask for em) and make them fit the wheel.
2. You car is stupid low to the ground, your guards are flared and your wheels are just the right offset that they sit flush with your guards, or just slightly out. If you were to run the right sized tyre for your wheels then you'd scrape the f**k out of them with your guard when you corner or when there's weight in your car. So you stretch the tyre, and the angle of the sidewall corresponds or tucks nicely with the flaring of the guard.
If you find a so called drifter that's running wide wheels, with stretched tyres and you can fit more than one finger between the top of his tyre and his guard, then go over to him and punch him in the c**t and tell him to either fit better tyres or lower his car.
Seriously... I'm sick and tired of looking at 4WD height drift style cars and posers running around with "omg sickkk dishh" wheels and $4000 paintjobs on their vertex/uras copy kits claiming to be drift superstars.