I used to work at a tire store and i can tell you EXACTLY why these idiots have the wheels/tires on the back.
They buy the wheels and have tires mounted on them, usually on lowered vehicles. Then they find out (as previously mentioned) that either because of the camber increase or just lack of clearance the tire/wheels won't fit on the front
OR
because those idiots buy universal wheels that happen to bolt up but have incorrect backspacing and can only put in on the rear because with the tires on the front they have problems with what i wrote above or because they physically can't turn around corners.
Whenever we sold wheels and tires we'd always recommend a test fitting of the wheel or 1 wheel and tire on the vehicle before the others to make sure that it'll fit. If it didn't the customer only bought 1 wheel or 0 because of the trial, once a tire is mounted to a wheel the wheels can't be returned.
This is my explanation, anyways.
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2005 Suzuki Hayabusa GSX1300-R
1980 Ford Thunderbird - 255 V8
ported heads, 5.0L ported stock headers, O.R. H-pipe and Flowmaster 2-chambers, dual roller timing chain
hi-po Mack Truck hood emblem
1985 Mustang GT 5.0L T5, F-303, GT40p, headers, off-road h, flowmasters, MSD stuff, etc.
Sold 02/06/04 
1989 Mustang GT ET: 13.304@102.29 mph (5-24-03)
Sold - 1998 Mustang Cobra coupe, 1/4 mile - street tires: 13.843@103.41 (bone stock)
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