The article doesn't say K2 is considered illegal, rather, it clearly states "legal, but aggressive." It does say the scheme is under investigation, but it's not declared illegal yet. The icebreaker one is.

My linking to the article wasn't just to point out these specific schemes, but more to just highlight the scope of tax avoidance/evasion. There are many loopholes that get abused, some get fixed, although not always in a timely manner.