People who are entitled to benefits should claim every penny they are entitled to. It's irrelevant whether it's "fair" to get what they do, they don't make the rules, it's up to the people who make the rules to make them fairer.
The same applies to tax. People who pay tax are entitled to do everything they legally can to decrease their bill. It's irrelevant whether it's "fair", they don't make the rules, it's up to the people who make the rules to make them fairer.
If people claimed benefits/paid tax according to differing conceptions of what is "fair" we would have chaos. What the law says should be the only criterion.
Tax avoidance isn't immoral. It's only over the last decade or so that politicians have started to try and sell the idea that it is.
Some quotes (selectively extracted from Wikipedia, which also has some that will suit the other side of the argument.)
No man in this country is under the smallest obligation, moral or other, so to arrange his legal relations to his business or to his property as to enable the Inland Revenue to put the largest possible shovel into his stores. James Avon Clyde, Lord Clyde, Ayrshire Pullman Motor Services and Ritchie v. IRC (1929) 14 TC 754.
Every man is entitled if he can to order his affairs so as that the tax attaching under the appropriate Acts is less than it otherwise would be. If he succeeds in ordering them so as to secure this result, then, however unappreciative the Commissioners of Inland Revenue or his fellow taxpayers may be of his ingenuity, he cannot be compelled to pay an increased tax. Thomas Tomlin, Baron Tomlin, in the UK House of Lords case, IRC v. Duke of Westminster (1936) 19 TC 490, [1936] AC 1.
Any one may so arrange his affairs that his taxes shall be as low as possible; he is not bound to choose that pattern which will best pay the Treasury; there is not even a patriotic duty to increase one's taxes. Judge Learned Hand, Helvering v. Gregory, 69 F.2d 809, 810-11 (2d Cir. 1934).