I would guess that stores see people who DO know what the abbreviations mean using it, and say "Hey, that'll save ad space!"
The complaint about "X-mas" meaning you're "crossing out Christ" is old (I first heard it when I was six, which I'm pretty sure is before you were born). It predates the modern PC bull sh-- attitude of "Must not offend non-Christians with Christian holidays". It is NOT some grand conspiracy against your Savior, God-boy.