It is legal, yet it is still tax evasion, even if one uses euphemisms like "tax optimization" and so on.

Whether it's legal or not greatly varies from one country to another. Some parts of their plan may be legal in isolation but huge corporations like Google are very much breaking tons of laws every single day (if only, privacy laws).

Also "tax evasion" means evading to pay taxes. It may have more specific legal meaning in some places, but the general understanding is if you're a company who does business somewhere yet doesn't pay taxes, you're doing tax evasion.

However, language isn't all about what's legal/illegal but also perception. Your average person is going to reasonably infer they use various countries to dodge taxes that they should reasonably be expected to pay if we take a holistic view of the world.