OK - well I'm not but let's assume - Why not :
- single out that this is not normal or moral (even if legal) and shop accordingly
- let the bad publicity be asserted again and again until it changes (by example, just tell every time you talk about Amazon about their tax rate)
- talk about the sociopaths ruling the place as such and isolate them socially until they change (you know, like in certain societies / countries it is not accepted to do some things but perfectly acceptable to do in others ? it works.)
until they pay like they ought to (even if they're not forced to legally)
and in parallel, talk loudly about changing the rules so that it becomes politically infeasible to not do it (then do it).