Why do Americans naturalize BEFORE giving up their U.S. nationality?

In the comments from the week before last I was discussing with another former American who is naturalized Japanese why Americans give up their U.S. nationality after acquiring Japanese nationality, as opposed to most cases (including the U.K.) where the situation is to give it up before getting your proof of naturalized citizen identification.

The reason for this is that American has two ways to give up citizenship: renouncement and relinquishment. When Americans become Japanese, they usually relinquish their U.S. citizenship, they don't renounce it. What's the difference?

• relinquishing means you willfully did something — that something being an expatriating act (such as swearing allegiance to another state [Japan]'s constitution and/or government) — and you willfully reported it — and you willfully did it knowing that doing so would cause loss of U.S. citizenship.
  
  If the last two "and"s and all the "willfull"s have you confused (wouldn't just doing the expatriating act cause you to lose it?) as to why they are necessary, the answer is history: the Supreme Court's rulings on Afroyim v Rusk and Vance v. Terrazas and the 14th amendment. In summary, because part of the 14th amendment protected African-American slaves' emancipation, the U.S. Supreme Court ruled that the part that makes it almost impossible for any American to lose citizenship unless they really wanted to applies to every American, not just former slaves. Vance v. Terrazas basically says that even if you tell another government that you are one of them and you don't want your U.S. citizenship, it doesn't mean squat unless you yourself actually tell a U.S. official personally and you know what you're doing.
• renouncing is for other cases — for example, forcing the U.S. to revoke your citizenship even if you acquired another citizenship without the original intent to lose your American citizenship.
  
  Because renouncing is considered far more serious and will most likely be impossible to appeal, it now costs $2,350 to process. Relinquishing is free.

This is why Japan lets Americans renounce after acquiring U.S. citizenship as opposed to before: by willfully acquiring Japanese nationality beforehand, it creates the prerequisite "expatriating act" (declaring allegiance to another country) that allows Americans to claim they legally relinquished their U.S. nationality as opposed to renouncing it. Relinquishing as opposed to renouncing is viewed differently in U.S. law in that a renouncer is more likely to trip flags regarding suspicion of giving up U.S. citizenship for the purpose of avoiding U.S. taxes. The Reed Amendment (which bans former Americans that renounced for the purpose of avoiding taxes) never applies to those who relinquish.

It is possible for an American to renounce as opposed to relinquish their U.S. citizenship after Japanese naturalization depending on what you say to the consular officer and write on the forms when you give up your U.S. citizenship, so it's important what you say during the interview with the U.S. consular officer.

What's confusing is that the U.S. uses the same form for relinquishing and renouncement; the consular officer determines from your answers as to whether it's renouncement or relinquishment. You'll know as to what the officer is thinking because if the officer thinks it's renouncement, they'll ask you to pay $2,350. If relinquishment, the processing is all free.

Renouncing does not necessarily mean you are becoming stateless. And it is possible for Americans to become stateless, though the U.S. makes it very difficult (e.g. they strongly discourage it and warn you) for one to do this, as one's travel / human rights could be restricted by lacking citizenship to a country.