[Big Bag of Chips]

The only reason I could see a teacher getting annoyed by the "wrong" brand is if the paper is so thin, and such bad quality that the child can't write on it without their pen/pencil going through!

My mother used to cover our copies.. in flowery wall paper. So you'd never know the brand - might be an option!

[Midnight Sundance]

Agree with what has been said, ordinary copies could be any brand at all. Maths copies can be a bone of contention at times when parents buy 10 large squared ones when child needs the small squares or the handwriting copy is the wrong size,
The brand would never matter in a school, getting the correct size would though and perhaps that is why a specific brand was named ,as a guideline, I doubt as a rule.