They definitely could.  I gave them each a bonus action and reaction instead.  I'm trying to find  ways to make interesting and engaging monsters without always having to resort to legendary actions.  It is still a work in progress 
Although, I have been thinking about a reduced legendary actions where there is only one or two actions.  That might work well for higher CR demons and devils.  Of the two, I think the Pit Fiend is a better fit as I see them as the more versatile fighter compared to the brutish balor.  I will give the Pit Fiend another spin when I get a minute.