The main issue here is the ambiguity of instructions issued by
their head offices. For instance, the old auntie at Woodlands Int
is notorious for stopping photographers and threatening to
confiscate the camera, while the staff at Yishun seem the least
perturbed. However, it seems that SMRT has recently changed their
policy regarding photography at their interchanges. Moreover, the
interchange is an open place, I don't see how taking photos at
interchanges accounts for trespassing.
Some green martians have this paranoid fear, some are actually
quite nice and understanding. The people at Tampines don't really
seem to care, after all they're used to bus fans barging into their
office to get service guides all the time, and when one talks to
them, they are very friendly and professional.