<i>But some of these tax avoidance schemes are only legal because tax authorities do not have the number of staff needed to investigate and prosecute all of them.</i><p>There is a difference between something that is legal and something that is illegal but difficult to prove. You seem to be conflating the two issues.<p><i>They use weird loopholes (which get closed, and which sometimes require repayment of unpaid tax) and bizarre corporate structures which only exist to avoid tax.</i><p>If tax law were not so onerous and burdensome, there would be no incentive to crease these corporate structures and to manage business in such a way.<p>There is nothing legally or morally objectionable about using the rules, written by your adversary, to your own advantage.