this might be silly but....

I really didn't like the strays and household pets in Sims 2 roaming around .. I had to get mods to get rid of them or at times I would completely leave out the pet pack when I had to reinstall the game. I was super glad that strays were basically confined to BB and yes you can have those lot traits if you want more cats or dogs to show up. I sometimes put out the lot trait when I'm looking for a pet to adopt.
So yeah I do believe it was for player choice and management. Although I don't use BB as often as other worlds because of the strays and it's a really beautiful world and you can always plop your sims down in that world to get your fill of strays.
Also now a days people don't let their household pets roam free as much in reality... it's just not done. I rarely let them roam free in the sims either unless they have a trait where I feel I should send them out and we have the option to call them back.