I wonder if there is something to do with which way the vent hole points. This may or may not be true since I haven't noticed every picture, but I know from two-three cars I've seen two M3 and one M5 that both had the vent hole pointing predominately toward the front of the car.
The vent hole on my car and on many other M3s I've seen point to the rear, i.e. toward the strut tower. I've run my car at Road Atlanta in 95+ humid weather, and STALKER was running his M3 there too and neither of us had trouble with power steering fluid leaking. The other factor perhaps helping is that my fluid level is just above half. When I checked the level at the track it seemed to me that by the end of the day the level had increased slightly.
I don't know if it's a blow-by issue (i.e. the vent needs to be open), an increase in level with temp or high load use, or the effect of cornering forces (do the autocross people have this trouble?) but for certain, some cars seem to be plagued with this and many other cars don't seem to have any trouble at all. I make a point of looking at M3s and M5s when their hoods are open to see if there is evidence of leaking and based on my experience (probably 20 cars or so), it's the minority of cars that leak (some BMW performance center cars leak and others don't).