Is tax avoidance always wrong?

In the argument about tax avoidance, people feel very strongly, yet it is hard to define wrong behaviour. We all know that tax evasion, being illegal, is wrong. But what tax behaviour is legal, yet wrong? Take a deliberately trivial example. Safety riding hats carry no VAT if they are sold as children’s hats. No law says that only children may buy or wear them, and no law limits their size. So it is commonplace for adults, without any dishonesty, to buy children’s riding hats for themselves to avoid the VAT. I struggle to see this as immoral. Is it just a matter of scale, then? Is it all right to avoid 20 per cent of the cost of a riding hat but all wrong to avoid 40 per cent of the cost of dying by passing wealth on early or through trusts? If so, why? On the other hand, one doesn’t like rich people ‘getting away with it’.