As the article points out they have an obligation to maximise profits. If there was a way to cut your tax rate from 20%+ to less than 5% and they didn't use it, it would be a breach of their duties.
It's amusing to see how effective industry lobbyists have been in shutting down any reform that would close these loopholes. Lets talk about slashing entitlements but completely stonewall any attempts to get corporations paying more tax...