Here's the MT law stating the legality of branding.
BRANDING AND INSPECTIONS 32.18.104
Sub-Chapter 1
Branding
32.18.101 HOT IRON BRANDS REQUIRED (1) Under the brand laws of the state, with the following exceptions only hot iron brands will be recognized by the department of livestock, brands enforcement division on all livestock for sheep, goats and swine and with the exception that freeze brands may be applied to horses, mules or asses. (History: Sec. 81-1-102, MCA; IMP , Sec. 81-1-102, MCA; Eff. 12/31/72; AMD , 1990 MAR p. 150, Eff. 10/26/90.)
Here would be my take on why freeze branding is not legal here. How easy would it be to paint over the discolored hair. very. When there is no scar no one can tell the difference. And then the rustler can turn around and hot iron the cow and bingo it's his. And then discolor the other freeze brand so it is not identifiable. Or if he is going to kill it and transporting it's a quick coloring and boom who's to say it isn't his.
Granted with hot ironing if someone has a simple brand (one letter or number)it can be altered also. So alway smart to pick brands that can't be altered.
I have thought about freeze branding our calves that go to the feedlot, until I found out it wasn't a legal identification. It would make the calf more valueable to the buyer and then with no hide blemishes valuable in the leather market.