Yes, they are perfectly legal.
This was one of Madonna's objections to the opening of her land under the Public access to private land. As it stands, the press and TV are allowed to venture onto public rights of way and cannot be stopped from taking what photos they like from them.
A few years ago the BBC commissioned a comedy drama called "Border Café" which was mainly filmed on Pwll Ddu, on the mountain near Blaenafon. They built a mock up of the Café to film all the outside scenes. Instead of "wooing" the locals and asking politely for co-operation, they hired a gang of burly security guards who attempted to close both the road and the two public footpaths every time they were making a take. They so incensed the locals, that they made filming very difficult for them by driving though at times of filming honking their horns as a protest at the director's approach to them.
The crux was that the Beeb attempted to obtain an injunction for temporary closure of the PRofW whilst they were filming. The judgment was that there was absolutely nothing preventing them filming of the said PRodW, but for the privilege, they would have to suffer the consequences of the open location, and the injunction was refused.
Needless to say, "Border Café" was a flop and lasted just one series.
As they were set up right over the top of the entrance to Ogof Draenen, Britain's forth longest known cave, (And being extended regularly) we were passing the spot quite often during the filming and we had many a conversation with the hundreds of staff milling around between takes. (Including the cast)
It was really funny though that they were far more interested in taking to us about the cave than we were talking about them !!
I would imagine however that someone attempting to set up a hot dog stall for example would fall foul of planning regulations at the very least !