Heated Blankets and Extension Cords

I have 4 stray/feral cats that hang out around my house, mainly in my garage and I wanted to put a heated blanket out there for them tonight. The directions say to not use with an extension cord but without one it wont reach where it needs to go. Is it really that dangerous or is it just one of those things companies put on warning labels to cover their ass? Before anyone says anything i can’t bring them inside for many reasons, but I live in Florida so it’s not going to get that cold, but cold enough to where I think they would enjoy a warm blanket.