I think that by using an inverted cross, they are trying to signify that they are martyrs (think about the disciples who were crucified upside down.) They're not martyrs. They are stupid, treasonous, obnoxious bimbos.
Its the symbol for the rejection of Christ. They arnt mocking those who boycott them. They are mocking the Son of God who was nailed on a Cross so they could continue to live in darkness. They may be to fricken ignorant to see it, but they are doing it none the less.