It also makes it easier to lie to the workers about exactly what the "card" is for.
The UAW keeps trying to unionize postdocs at the University of California. They do this by coming around to their offices, one-on-one, pulling them out to the corridor, and pressuring them to sign a card which they say is "to get more information" or something. Many people (and remember, an awful lot of postdocs can barely speak English) just sign the card to get them to go away, without realizing that they've just officially voted for unionization.
The UAW keeps trying to unionize postdocs at the University of California. They do this by coming around to their offices, one-on-one, pulling them out to the corridor, and pressuring them to sign a card which they say is "to get more information" or something. Many people (and remember, an awful lot of postdocs can barely speak English) just sign the card to get them to go away, without realizing that they've just officially voted for unionization.