Because I absolutely despise when a strange dog comes up to me and is not controlled. I don't care how well trained the dog is, I do not want an unrestrained dog in me or my dog's space because my dog does not do well with other dogs being so close to her.
I'm sure most dog's are typically fine and trained properly, but I do not want to risk my safety or my dog's because someone else was walking their dog and it was off a leash.
Typically it's on campus property, which doesn't have signs to leash your dog, but it's a decently crowded place during passing periods.
I stay far away from dog sports, so I have no clue about those ideals and rules. I just know where I've gone has resulted in me suddenly having unrestrained dogs run over to me and I am not a fan of strange dogs