Look at her, of course.

While there are exceptions, it usually takes a hell of a lot of effort to use a captured warship in combat. If the US managed to recover the Akagi(which was debatable considering how wrecked the ship was), they would have to significantly repair the ship and then pretty much remove all guns, all machinery, and most other gear(pretty much everything that is not structural) and replaced it with US equivalents to be able to use the carrier for any long time.

What they could do, however, was look at her.

Even heavily damaged, a lot of intact equipment can be found in a warship and a lot can be understood concerning her structural form and compartmentalization. If the US Navy wanted to figure out how to most easily sink a Japanese aircraft carrier, one of the best ways of doing it would be getting a captured carrier and studying it, including its apparent capabilities and weaknesses.

And of course, then there is the problem of propaganda. Getting the flagship of your carrier fleet sunk is bad enough: getting her captured is downright humiliating.

Thus, it was general practice that if one of your ships was a lost cause, you evacuated the crew and then scuttled it: to give an example, that was the fate of the German battlecruiser Lützow at Jutland. When the flooding could not be controlled Germans evacuated the ship and sunk two torpedoes into her broadside. A stricken ship was better off in the bottom of the sea rather than being paraded around by the enemy and definitely better off than being cut open by a group of naval engineers looking for weaknesses.

Admittedly, most ships that were in the situation of the Lützow or the Akagi were too badly mauled to recover. They most probably would go down anyway. But ensuring they definitely go down only takes a couple minutes and a few torpedoes: no reason not to.

Follow up: Did the US realize at the time that the carrier was scuttled? Or was it only until after the war that we learned this?