Over the last few years Sustrans have repeatedly tried to sign the Greenway to ensure that it is safe and pleasant for all users.
As can be seen from the signage at the Ripley end of the Greenway, it would seem that someone seems to think that the Highway Code (Rule 56 Dogs - Do not let a dog out on the road on its own. Keep it on a short lead when walking on the pavement, road or path shared with cyclists or horse riders) does not apply to them.
Vandalising signs does not change the Highway Code. Responsible dog owners abide by the code, irresponsible and inconsiderate ones do not.