I think the argument is stronger than that. It's not that tax avoidance is morally justified, it's that corporations have a fiduciary responsibility as stewards of their shareholders' investment to maximize the return on that investment (within the bounds of what the law allows). That includes reducing costs and, yes, that includes reducing tax burden.

Tax avoidance within the bounds of the law is not just morally justified - it's morally required.

(For the record, I think tax avoidance is a major problem, especially because large corporations end up being the only ones who can afford the accountants, lawyers, and international subsidiaries to facilitate it. Small business ends up getting shafted. But the way to solve it isn't to whine about Apple doing what Apple is supposed to do. The fix is to reform the tax code.)