Dog owners........?

i find it disgusting as im sure most people do that some dog owners take their dogs for a walk on a PUBLIC footpath and then blatently allow them to foul on the pavements......! it irresponsible and a danger to health. i wonder why there are now laws in place whereby dog owners can be fined for allowing this to happen but yet who is going to stop them....am i to make a citizens arrest..... . why bother to have these laws if its left to the public to report the offenders.