As stated, the laws is the laws. Honestly, the telescoping stock rule bothers me far more than the flash hider rule. Think about it, you can have an M4 telescoping stock on your rifle as long as it's pinned in one location. It doesn't matter what location on the tube that you pin it, it doesn't matter if it's all the way in or all the way out, just that it can't move once it's on the rifle. So basically what the means is, you can't have a stock that makes your rifle more comfortable to shoot in different situations or from different positions. IMO It makes far less sense in terms of law and public safety, than any of the other "assault rifle" rules. There is absolutely no argument one could make, that lends to defending the idea that a telescoping stock makes a rifle of any sort, more of a concern for public safety.
I don't really care about flash hiders. My problem with that part of the feature rule is that I'm a tinkerer and enjoy changing things up from time to time. I'd rather use a compensator anyway as a comp is more effective for me in my uses than a flash hider. I can't remember the last time I was out shooting my personally owned rifle at night and had a high concern for giving away my position with muzzle flash. I can however, think of a million times when I wished I could adjust my stock to fit a position more comfortably.