The question is how much tax they should pay. To be fair and avoid double taxation, you'd roughly want to tax revenue in the US minus costs made to create that

The question is how much tax they should pay. To be fair and avoid double taxation, you'd roughly want to tax revenue in the US minus costs made to create that revenue. That quickly devolves into an endless fight to define exactly what contributes to US revenue, and which costs are made to create the US revenue. It's easier to workaround that endless fight by ensuring that it doesn't matter where tax is paid.