Actually tax shelters are fraud...
That really isn't true in a general sense.
It depends on a bunch of factors. If Ronaldo was hiding the income he got from Real Madrid to pay the Spanish state less in taxes, that would be fraud. But this leak refers to money he made off international commercial dealings, outside of Spain, and that money going into these offshore dealings. It's "shady" and the intent is barely covered, but at the end of the day, it's just what about every rich guy with a skilled accountant does - move just on the line of what the law allows, and the Spanish tax law isn't exactly the harshest. I don't (yet) see this being much of a scandal, to be honest. This is another confirmation of rich people paying top tax lawyers to use the existing rules to their biggest possible advantage, which is barely news, let alone a scandal. The sources I saw rather clearly treat it as something they deem shady, but very possibly within the leniant Spanish tax law, and merely morally questionable.
And I say that as someone who does not like CR one bit and will happily use this (in some way) in a debate with my Portugese friend who mainained that CR was a much better human being than Messi for the latter's dealings.
Also, pay is publicized in the US to prevent this. I can tell you how much any Laker player makes. Exactly how they got to the team. I cannot tell you how much every Liverpool player makes and how much transfers exactly cost. When they are exactly paid. That's the problem. There is too much opportunity to steal.
That isn't really the issue. This isn't about money CR made off Real Madrid, this seems to be about international endorsement deals outside of Spain. Noone would know about that, just like noone knows about what any Laker player makes of some Japanese commercial. The opportunities are about the same. Trying to evade taxes from your national employer is just about the dumbest thing one could even try to attempt (in the EU, at least), and I'm not even sure there is much of a way to attempt it, given how social insurance etc. work over here, and it would require the employer's collaboration, at the very least, which there are no signs of in this case.