As a former union shop steward, I have seen the same thing you describe but it's been my experience that the reason employee's get away with being lazy is inconsistency in the administration of discipline & rules.

It's kind of hard for management to crack down on employee A when you let employee B get away with doing the exact same thing. One thing a union does is force an employer to be consistent for better or worse.

BTW, all of the union business agents I know HATE getting into a situation where they have to represent a lazy, dead beat employee when there are others being wrongly disciplined, terminated, etc... without just cause.
Regards
Bill R-4581