Because the law does not work that way. Japan values property in yen, and the law states that you must be paid back the value of what you lost at the time you lost it.

This is true in most countries -- in the United States, if the state for any reason seizes your coins without legal reason, and has time to auction them off before you can prove they had no legal reason to take them, you will be paid back in dollar for the value of the coins on the day they were taken. The state does not owe you the potential increase in value between that day and today.

Where it gets interesting is what would happen if the reverse was true. Let's say your coin gets taken without legal reason in an illegal search today, with the price at $100. Two weeks from now, they get auctioned off after the price collapsed to $80. Four months from now, when your lawyer proves to the court that your coins were legally acquired and there was no basis to the seizure, the price is at $1. What does the government owe you?