GM is a factory with a semi-skilled labor force. They produce very expensive goods. Because of this, they don't produce at maximum output all the time. If sales numbers slump one month, they cut production. To cut production, they have to cut their workforce. When they cut their workforce, they don't want to fire someone that has a lot of experience because they may just need him/her next month when sales pick back up again. So they pay this person to keep up a relationship until they need him/her back at work. They may not even pay this person his/her full rate.

Consider that while one car's sales are dwindling, another's might be picking up. Well, a person putting doors on a Chevy Malibu can probably do the same thing on a Chevy Trailblazer. In this case, the worker isn't paid to be idle, he/she just switches car lines.

Even if the worker is paid to be idle, you shouldn't blame the worker or the union. A contract requires two parties. The auto manufacturers made these agreements with the unions, knowing the consequences full well. Why blame a person for wanting nice pay and benefits?

Also, if the auto manufacturers wanted to renegotiate to save themselves, the unions would support it. In the past, the US auto manufacturers have renegotiated union contracts to cut pay. After all, what good is a union if its members have no jobs?