My first thought is that they might not have been cruising, but I also remember someone telling me that a common technique is to bring your dog to a cruising area because it lends credibility to being there in the first place (Cop: "Why are you here?" Cruiser: "Just out walking the dog!"). So much so that in place of 'cottaging' as they say in the UK, that some guys call it 'dogging'.
I guess if you have someone at home who doesnt know you cruise, taking the dog with you might also be a good excuse to take a long walk in the local woods.
I also recall a good story I read once about a truck stop encounter that featured a guy who had his dog trained to wait out of sight so that if they got stopped, they could pretend they'd actually lost the dog and with the right call out, the dog would come running to back up the story.