Remember "tax minimization" is legal, prudent, and a fundamental duty of company officers in these times of "maximise shareholder value" being the primary and legally mandated goal of a corporation.

"Tax evasion" is crossing over a not very well defined and continually shifting line - and is illegal.

At the moment, at least from where I see the world here in Australia, all the "global tech giants" (and mining/resource ones too) are doing a significantly better job of navigating the shifting tax rules across various national tax jurisdictions to minimize their tax obligations - than the governments and bureaucracies are doing of collecting "fair" taxes from them. Our local popular press is full of stories of FaceBook/Google/Apple paying zero tax in Australia because they can structure their invoicing/payments to and from different parts of the same global corporate entity – in entirely legal ways that result in zero tax owing in Australia.

I'd think less of Dropbox's senior management and advisor team if they weren't trying to take advantage of the same loopholes.

It's "Modern Corporation 101" grade stuff - "Maximise shareholder value by whatever means won't put the company directors in jail. Don't talk about ethics or fairness or duty to society (except in the Marketing Department and advertising). Talk about duty to shareholders and what's close enough to technically legal that our lawyers will sign off on it."