Because pure e85 will ruin the motor lol. Plus the engine is not emissions tested for a mix of e85 and many other reasons why bmw is not going to state (car will be fine on mixes of e25 or below). Also why they don't comment on race fuel.
The engine and fuel system cannot handle probably e40 or higher for any length of time. E25 you are still pushing your fuel system to work harder and that alone is a reason bmw would not say run this. Your average person does not understand or is able to log his afr, timing and parameters to ensure your ECU is compensating properly by adjusting fuel trims appropriately etc.
Like any performance item it takes some understanding to do it properly and if you understand the way an EDU works, how fuel trims compensate for ethanol and how to monitor proper parameters than it is totally benign. The fuel lines etc in all modern cars frankly are made to tolerate ethanol as ethanol is in all our gas and different countries have even more ethanol than we do.
Don't do it if you are not comfortable but e25 or about 3 gallons a tank has noticeable benefit with the parameters are monitored and running perfectly. I am willing to accept a fuel pump and injectors that may go out 25 percent earlier and its an acceptable risk to me. Aside from that if done properly and monitored there is no risk to an engine.
Its another story if a guy just dumps e85 and blasts out of the station at WOT and the car runs so lean that the engine explodes. Most guys who do this know they drive for a little while first, make sure the ecu has time to adjust fueling. The idle will go crazy a bit at first and that is the ECU adjusting fuel trims to meet AFR goals with the no fuel mix and once all settles out and one monitors atleast once to ensure the car is healthy enough to make these compensations than they let loose. The average joe is not going to understand all this.