With no legal sanction, they get to walk. There is, at least, the chance to get the BTP involved with a legal framework.
The railway already has the right to boot people off if they don't make use of ticketing facilities. That was the original whole point of Byelaw 18, before a minor grammatical change in 2005 made it a nice little money-spinnner for the TOCs!
Penalty Fares don't give the TOCs any more physical rights - and the only time the BTP would get involved is if there was an offence being committed. Which would come, unsurprisingly, from laws like the Byelaws or RoRA, neither of which have any dependence on Penalty Fares!
Don't get me wrong, I think there is a time and a place for Penalty Fares. But the indiscriminate manner in which they seem to be spreading across the network, especially on lines that are well outside the kind of line they were originally designed for (intensively served, primarily DOO, surburban lines), is making them dishonourable.