Most governments are elected on less than 50% of the vote. The government spends the taxes they raise from us. That means most people are handing over money to people to spend on their behalf, when they don't agree with the policies of the people who are spending it. This is a major reason why people attempt to legally avoid paying taxes.
When it comes to business taxes it's even harder. Most businesses have to a) compete and b) deliver the best return possible to their shareholders. If one competitor is legally avoiding taxes then others will find it difficult to compete, let alone deliver the best return possible, unless they do likewise. Sure they could try to use PR to persuade good-hearted customers that they should buy from them rather than the tax avoiders, but when it comes to it most people and other businesses will still buy from those tax avoiders - Amazon, Google, Starbucks ...
By the above logic, for a union under a Tory/LibDem coalition, tax avoidance makes perfect sense. They don't agree with the government, so they go all out to ensure they hand over as little as possible to that government. Even under a Labour government, they have a duty to their members to spend their money wisely and not on things that they did not need to spend it on, such as paying more tax than they had to.
The Government isn't a charity, but in a perfect world, people and businesses who avoided paying large amounts of tax would also donate large amounts to charities and other socially beneficial causes that they did agree with. Unfortunately it's not a perfect world and so this doesn't always happen, but it's not uncommon. For example, Google gives a lot to charity - although the charity isn't necessarily in the country where it avoided paying the tax ...