Why do helicopter rotors have constant section and angle of attack?
Helicopter blades do have twist. The relative air speed increases from blade root to blade tip, and therefore on an untwisted blade lift would increase quadratically from root to tip. Or going the other way, lift would reduce quadratically from tip to root. Blade twist fixes this: it recuperates the lift generated on the inboard section of the rotor.

From Ray Prouty, Helicopter Aerodynamics Vol. 1., page 248:


  By twisting the blades to reduce the lift at the tip, the flow pattern can be made more uniform and the hover figure of merit can be improved by up to about 5%. Since this may represent a 20% increase in payload and since twisted blades are about as easy to manufacture as untwisted blades, there is a good reason to use twist - and all modern helicopters do.


Due to the aerodynamic lift equation, ideal blade twist would be non-linear: again, would vary quadratically with blade span. But this ideal twist is only ideal in the hover situation, and definitely not ideal in the factory when the blade needs to be produced. 

Pouty's article is very much worth reading, and further mentions:


High twist produces vibrations by the centre of pressure traveling a lot during rotation.
Negative blade twist (tip angle lower) is good for retreating blade stall but bad for autorotation, which would be helped by positive blade twist.
Optimum twist differs for hovering In Ground Effect and Out of Ground Effect.