Are people allowed to let their cats roam the streets ?

It depends on the area you live in. In my area the leash law applies to cats so no legally you are not allowed to let your cat roam at large. In others areas there are no containment laws that apply to cats. Even in those areas however you can still report it if the cats are damaging property, cause a hazard etc. With a large population of cats such as this you can also contact animal control and request they set live capture traps on your property. You will simply have to monitor the traps (typically check theme every x number of hours) and call animal control when there is an animal in one of them.