Are the tax loopholes intentionally written into the tax code to allow for the kinds of shenanigans that Amazon (and many other major companies) pull with their taxes? Or, do these companies spend millions of dollars on lawyers and accountants to create, find, and exploit portions of the tax code that were written with a different intent?

If the answer is that it is intentionally written into the tax code, then the blame is on the lawmakers.

If the answer is that these companies follow the letter of the law but not the intent of the law, then the blame is on these companies (although it seems a case of 'fool-me-once' / 'fool-me-twice' and there is no excuse for leaving these loopholes open).