BMW parts are expensive.
Your answer doesn't help understanding how the article finds the average BMW maintenance cost to be higher than that of the BMW model with the highest maintenance cost. Unless brand costs are not calculated as a mean but something else very different, which is not explained.
Apparently, the 328i has a relatively common oil-leakage problem. http://repairpal.com/bmw/328i
Don't have the labor costs for a valve cover gasket, but they could be sizable. Scrolling down on repairpal web page above, apparently they can cost up to $1000?
No wonder BMW 328i is in the higher range for maint. costs!
The problem is that paying for parts and parts replacement piece by piece is significantly more expensive than buying a new bike (or a car) as a whole.