>>> the term ought to be 'dogs under full control'
It is (or damned close to it, anyway):
"Every person in charge of a dog on the access land shall as far as is reasonably practicable keep the dog under close control and restrain the dog from behaviour giving reasonable grounds for annoyance".
So an obedient dog, walking close to heel (but not on a lead) is fine. The only time that a dog needs to be on a lead is when a Ranger has specifically said so:
"Every person in charge of a dog on the access land shall, as far as is reasonably practicable, comply with a direction given by a Ranger or other officer of the Authority to keep the dog on a lead".
http://www.dartmoor.gov.uk/about-us/who-we-are/byelaws
I agree though that it seems odd that the hunt seems to be allowed to ignore the rules. Their own website includes advice to members and guests about displaying good manners but I can't see anything about respecting the moor and its wildlife:
http://www.dartmoorhunt.co.uk/terms.html