I think you are being unreasonable, but understandable in light of the bad publicity. I think so long as the owner is confident that he has the dogs under control, seeing them off lead is OK. We were in greenwich park on Saturday and we walked around the corner and there was two rotties and a GSD, oh, and a lab off the lead - i was with DD, so i thought best to keep her close. Then i saw that they were with owners who clearly knew what they were doing and were happy for the dogs to be off, i relaxed. The dogs showed no interest in us whatsover so i felt no threat at all.
I have owned two rotweillers and i most certainly did not want them as a look at me im hard status symbol. They have a steady temprement, and TRAINED PROPERLY they make good family pets. So to say all rottie owners are macho idiots is just plain stupidity. I will probably have another rottie one day, but not whilst DD is young. But the same goes for a labrador.
As for the dogs being off lead, i dont agree with you im afraid, is it simply because of the breed? It sounds like it is dog walking area and i think the owner is entitled to let his dogs run free as anyone else. I used to let my first rottie off most public places, because he was very obedient and would ignore people, unless encouraged over for a stroke (surprisingly lots of people like rotties - they are an impressively handsome dog). My second rottie was not so easy, he was a rescue and a bit nutty, so i would either only let him off in the woods, there would be the occasional family but they would be expecting lots of dogs, i used to let him off the lead in the winter on the beach, but not in the summer, because he was stupid and wanted to say hello to everyone and that is not acceptable. The same with my jack russell, he has no brains and thinks the world is his friend, so i keep him on the lead in populated areas, more so than the rotties actually as he has crap recall.
I would expect the dogs of course to be under control enough that the owner could call them back to him if a family wiht small children came along, lest they bowl the children over like skittles.
Please give the rottie a break, get to know a nice one, you couldnt hope for a more affectionate, calm and loyal pet.