If that's the issue then it seems to me that the solution is greater enforcement at the departure port. Maybe this has been covered elsewhere, but I noticed a change in tax free purchases as of last year. It used to be that the retailer would staple a receipt to your passport and this was collected at departure Immigration when you left Japan. The retailer also made a hanko impression partly on the receipt and partly on the passport page, ostensibly so that you removed the receipt yourself, Immigration would know. Now, it seems to be all electronic, with the retailer entering the sale on a gov't website at the point of sale. If you've made duty-free purchases, they now have a passport scanner at the Customs desk just before the Immigration counter. You scan your passport, and it decides if you need further inspection. I think for most people, it just beeps and there's nothing more to it. But if you, say, bought 100 iPhones tax free during your stay, it would probably alert and the Customs officer would want to see evidence that the phones are actually being exported, or presumably you'd have to pay the tax before departing. The passport scan at the Customs desk seems optional with most people just walking by, and I guess that what happens is that if you're flagged for inspection, your passport would alert at Immigration and you'd be sent back to the Customs desk, so the pre-scan at Customs allows you to avoid a wasted wait in line at Immigration. At least that's how it seems to work to me.

In any event, they seem to already have a mechanism in place to catch people cheating, at least at a large scale. Perhaps I'm missing something.