Because it looks
cooler to fire a rifle/subgun with an underfolding stock folded. When in doubt, that's the answer to everything that doesn't make sense about guns in movies. Duh.
To play devil's advocate for a second...
(1.) How common is this in movies? Have you done a survey of every movie/TV show featuring an AKS-47 or AKMS derivative, and calculated the proportion of them that show BGs firing with the stock folded? If not, can you really say that
every movie has this problem?
(2.) Doesn't this happen in the real world, too? I've seen pictures of AKs captured in Iraq, and I've seen many that are actually
missing the stock completely (regardless of whether the gun was a fixed or underfolding model). Which suggests to me that maybe those guys don't care much about accuracy. And you know what they say about life imitating art?