If people who do not dress in a certain manner were a protected class under the law, then "yes" those shop owners would be guilty of discrimination. But as of today, the sign can stay without risk of penalty.
@Right_in_Virginia This is why I noted that discrimination has an original meaning which has been obscured, and it doesn't involve politically protected groups of people.
Definition of discriminate
discriminated; discriminating
transitive verb
1 a : to mark or perceive the distinguishing or peculiar features of
Depth perception may be defined as the ability to appreciate or discriminate the third dimension … —H. G. Armstrong
b : distinguish, differentiate discriminate hundreds of colors
2 : to distinguish by discerning or exposing differences : to recognize or identify as separate and distinct discriminate right from wrong; especially : to distinguish from another like object discriminate the individual voices in the choir
intransitive verb
1 a : to make a distinction discriminate among historical sources discriminates between literary fiction and popular fiction
b : to use good judgment
Shop owners who put up the "no service" sign are making a distinction between people who are appropriately dressed, as distinguished by the owners. Bakeries are making a distinction between gays who want any form of baked goods and a wedding cake. In both situations, customers might be turned away, but they can easily find service elsewhere.