1. The orignal G'zeira was to prohibit selling "B'heima gassah" etc.
2 The local Minhag was to extend that to B'heimah Dakkah too, sort of a g'zeriah onto a g'zeirah, See Bartenura there.
3 Alternatively - it's simply a broader re-definition of the g'zeirah.
A. Local Minhaggim as Humra are often Harchakot for Issurim, s'yagim
B This helps to explain the Minhag of not eating fowl during the 9 days