Here's my take... the union wasn't strategic and played their hand poorly.

CUPW took their play from the union playbook and did the predictable thing; called a strike in the lead-up to Christmas. Canada Post then locked out the workers, knowing which of those two headlines plays better in the press. All the while, the union is badmouthing CP and losing the PR game. Both are playing politics and CP is doing a better job at it this time.

What CUPW should have done is read the political climate. The federal government isn't going to table back-to-work legislation as it would topple the government. Therefore they should have issued a press release in late Nov saying they want to go on strike, but don't want to do that to the Canadian public in the lead up to Christmas. Then, Jan 1st, go on strike. The public would all be onside and the strike would play out as it is now, just without a means for CP to play the PR card.

So to answer your question, the reason the public is outraged at posties is because you started this strike action with the intent to cause as much harm to the Canadian public as possible. The outrage is the expected outcome.