Probably the novelty or rarity of printing B&W makes them think they should
charge more. Plus they may have to interrupt the normal colour flow to flip
the switches into B&W mode while they put your job through. But really, if
you are shooting B&W these days why are you taking it to a mini lab? The
best you can expect from anything a mini lab does is to use the output as a
"proof" copy. I have a scanner and photo printer but they pale in comparison
to what I'm able to achieve by printing my own B&W at home.

You are mistaken in assuming that the price is dictated by cost. Once
the costs are payed, price only depends upon what the customer is
willing to pay. And they pay more for rarity, exclusivity, or anything
else different from the usual. So, vote with your wallet, and tell the
shop you are not willing to pay more for the same work.

Gilbert