If I recall properly, the original design for BO-105 fuel system stemmed from a desire to meet Certification Requirements to allow for a Fuel Dump capability to lighten the aircraft following a single engine failure. That required the use of the Supply Tanks to isolate a certain amount of fuel from being jettisoned. The EC-135 is merely a follow-on to the 105 series aircraft.That is why the Fuel system is the laid out the way it is.Sometimes that results in "Unintended Consequences".
Sasless,
The design of the fuel system is based on the criteria as outlined in CS-29.953 Fuel System Independance.
Although the 135 is a CS-27 (Small Rotorcraft) certified helicopter; it is also certfied for CAT A Ops. For this, it also has to comply with some paragraphs from CS-29 (Large Rotorcraft) requirements.