It was twenty years before GM saw competition from the Japanese (in the wake of the oil crisis), and it's hard for us to see six months out in our own companies. Any rational shareholder, were they so smart, could have dumped GM in 1973. And, really, 1950 to now is two whole generations (generally defined as 30 years). Fiduciary duty means a "reasonable person" standard. While, yes, it would have been better now if they had the government take on these obligations then, it's clearly not unreasonable for GM to have decided otherwise.
Second, and more interestingly to me, is that it was in GM's interest to bear health care costs. GM needed to attract and retain skilled labor (which is why they compensated them so well in the Treaty of Detroit). Bearing healthcare costs, as a flush company with two full decades of prosperity ahead of it, provided a competitive advantage in attracting and retaining labor versus other fields that these workers could have gone into, such as plumbing, construction, or the like.
I'll leave the historical nature of the Big Three's political biases to others.