[Goose]

How did the mere wearing of masks become contentious? 

It's a very overt sign of being forced to do something that you're not used to.  Some people are more resistant under such circumstances than others.

Also, perhaps masks have had a negative connotation in our history, being linked to robberies and such.

To be precise, it goes back much further than that. During feudal times entering the house or land of another feudal entity with your helmet visor down (or your chain mail hooked up to your helmet if you were a man-at-arms) was an act of war and you could be killed on the spot. These things do carry over it seems.

[Goose]

How did the mere wearing of masks become contentious? 

It's a very overt sign of being forced to do something that you're not used to.  Some people are more resistant under such circumstances than others.

Also, perhaps masks have had a negative connotation in our history, being linked to robberies and such.

To be precise, it goes back much further than that. During feudal times entering the house or land of another feudal entity with your helmet visor down (or your chain mail hooked up to your helmet if you were a man-at-arms) was an act of war and you could be killed on the spot. These things do carry over it seems.