Credit card/cash questions for travel in Canada

Interestingly enough, just a few days ago, I saw a sign in one of local stores that I had never seen before. It said NO US cash accepted. I asked if this meant just the bigger bills, 50s & 100s as you do often see signs for that, but no, this was for all US bills, as they said they had had so many forgeries, even the smaller bills.
The Canadian bills, being so coloured and plastic are probably too difficult to forge, so maybe all the bad guys, including the Can. ones are turning to US bills?