
I would think that as a dog bin is provided at the entrance of the field then one must presume, as you did, that it is ok for dogs, I would have walked mine there if I had seen it ;)
You should have asked the guy that approached you, where the "no dog" signs were ;) Thats what I have done in the past when I have been shouted at for walking my dogs on leads.
I suppose they must get fed up with dogs fouling the field, as we all know there are some very irresponsible dog owners out there who don't give a t**s where their dogs go.
You probably were in the wrong place at the wrong time, perhaps he'd had a row with someone and took it out on you :D Don't let it bother you.