Julian Cope presents Head Heritage

Paddy Power about Uffington -
“We didn’t ask permission because we knew the answer would be no. But no harm has been done..."
http://www.oxfordtimes.co.uk/news/9579762.Horse_stunt_is_a__desecration_/

So they were sure the publicity was worth the subsequent telling off and they're sure no damage has been done.
And they're probably right in both cases as the Ancient Monuments Act provides penalties for "damage" not disrespect or miniscule surface holes caused by tiny tent pegs.

NT probably didn't make it clear enough they are against stunts (and have taken money for them previously) and will do no more than moan now. EH et al aren't likely to formulate a Code so if people like us are serious about opposing this stuff it's up to us to formulate a Code and force EH to take it seriously.

(I personally don't believe Paddy Power would necessarily have ignored a Code, if accepted by all the organisations. The reason they did what they did was because the message against it simply wasn't clear enough - and in NT's case, non-existent).