i think its due to the misconception of government thinking that arts stream students are generally weaker than science stream students.
anyway u can always appeal to ur school administration and see wat happens.

AFAIK u must confirm wat subjects u are taking upon registration.
u can simply choose any subjects u like as in SPM.
this is because one or more of the subjects will overlap each other during STPM exam.
another thing is that it makes it harder for the teacher to put up a schedule to teach all ur rojak subjects.