It's depend from country to country and theirs laws.

In mine, we have to pay tax only if we buy from a person living in my country. So mostly it isn't a strict law to actually pay but most of them never pay. And also there is only tax if we purchase 20 bitcoin or more on a single day so many people just purchase 19 bitcoins daily.
Some of us buy it with Paypal to hide it. There are laws but aren't so strict if we purchase with credit card or bank transfer no tax.