I think the apostrophe is because it's 'understood' to be short for 'butcher's shop'. Same as the ditty about the tablecloth which used to belong to my grandmother...it was my grandmother's (tablecloth understood)
That wouldn't sound right around here. In the UK we'd usually say at the butcher's. It's short for "at the butcher's shop" - and the "shop" is generally dropped. "At the butcher's shop" is also a more direct translation of "Chez le boucher."