It's one of those union wet dreams, they claim it allows companies to scare workers away from unionization before the secret ballots when allowing card counts instead will allow unions to bully workers into signing up.
Personally my biggest problem with unions is they value time worked above all else. Someone who has been employed 20 years is nearly impossible to fire, even if they happen to be the least productive, most overpaid, gloomiest person in the building.
I've noticed this puts people in a bit of a bind, either they can leave their job of 10-20 years for something that may be more enjoyable or better paying and lose all their seniority and security or they can stay put at the same old boring, repetitive job, which makes them more pissed off as they count down the days till retirement. It's really sad how many people I know who choose option B.