One thing to note here is that the closure is a costly, credible, verifiable signal of commitment to their principles. Unusual or questionable principles, perhaps - few other Christians feel the need to close on Sunday - but principles nevertheless.

Suppose B&H or Chick-Fil-A suddenly changed their policy to be open, which is easy to verify (just see if you can buy something on that day). What do you think the reaction of their customers would be to this greater convenience and no loss of quality? Would they celebrate? I think the reaction would be highly negative - everyone would panic that they had sold out and the beancounters were now in charge and the quality was going to go downhill while the prices go up.

Precisely because the policy is so economically foolish and is not a marketing stunt (notice no one is clamoring to imitate them even though it's trivial: just close on some day) and would be one of the first things to go under new (ie. normal) management, it proves that they don't care that much about maximizing their profits but other things.