Because the bitch in season is the responsibility of the person who owns her. She is NOT the responsibility of anyone else who might happen to own an intact male dog in her n'hood. As someone else said, a wise person does not EVER leave a bitch in season unsupervised outdoors. NOT EVER.
I have 6 ft tall excellent fencing with locked gates. I DO NOT TRUST ***ANY*** FENCING TO KEEP A BITCH IN SEASON. I DO NOT EVER leave a bitch in season out in my yard, NOT FOR 30 SECONDS.
If the person with the loose dog is liable, it would only be for the broken leash law.
A bitch in season is generally considered as an attractive nuisance. There are laws on almost every animal control book about how bitches in season must be confined.
If one is ignorant or careless enough to leave a bitch in full bloom season out unattended in a yard, I say they made their bed, they should lay in it.
JMNSHO....