Why do car washes put covers on the back wipers of suv's and vans?

[gsxregal]

I never understood why car washes put covers on the back windsheild wipers of suvs and vans. If the carwash is going to rip the wiper off its going to do it regardless even if theres a plastic bag on the blade. (mikes carwash is the only one i see do this) The second part to this question is why do people leave the bag on their blades?? Ive seen dirty cars that obviously need a wash with these things still on them.

 

[RapidFire1]

But the odds of the brush not getting caught on the wiper favour the wiper with a smooth cover over it that won't catch the individual bristles. N'est pas?