They frequently already are. Even here in Canada[1] where any kind of "carry" is generally not allowed. Shockingly, thieves often don't care about gun laws though.

[1] https://www.cbc.ca/news/canada/toronto/toronto-police-cash-r...

Your link refers to a single robbery which explicitly requires that you threaten another individual. "Thieves" encompasses far, far more than robbery, crimes where you needn't threaten someone else and carrying an illegal firearm becomes a liability.