Since I'm not the OP I can't answer on their reasoning but I also don't like or allow my horses to turn in when I'm lunging them. My reason is that I hook the lunge line up by clipping it to the bit on the outside, up over the poll and through the bit on the inside. If they turn in and flip directions then my lunge line is on the wrong side and it's more difficult to control the horse. Plus I'm always worried they'll step too far in and get tangled in the line. I certainly don't want a line wrapped around their legs and attached to their head. I like to make them wait for me while I gather the line up, switch sides and then send them off the other direction. For me, it's a safety thing.
Good job OP for establishing respect and enforcing the rules. It sounds like you're doing exactly the right things. Keep up the good work!!
I appreciate all the great advice from the members here. Good community -- I think I will stick around a bit :p