There seems to be a lot of muddled posts about tax avoidance - perfectly legal, tax havens - been going on since taxes were imposed, and tax evasion - criminal offence.
It's not just the rich who evade taxes. Think of the tradesmen who ask for cash to avoid VAT etc.
Successive governments for decades have failed to tighten up loopholes or provide the money for the Justice system to investigate fraud. There is nothing new under the sun.
Aside from morality and justice, governments also have a responsibility to tax efficiently. One bizarre example I saw recently was Apple issuing US Bonds, when it is sitting on a 40 billion USD cash pile. The money is in the bank off shore. For Apple it is more efficient to pay interest to US bond holders than to repatriate it's own funds and pay US taxes. The yeild on the bonds is low, as lending money to Apple, given their cash balance is almost as good as lending to governments. From the point of view of the US economy though, this is money that could have been invested elsewhere and produced a tangible benefit for the US economy. After all Apple didn't need to borrow the money it already had it. I am not making any value judgement here, just pointing out the interdependencies between state and capital.