One point of view is that this is related to spicy, with 2 legs coming down. Another is that it is more like "stand" on top of "tree". Is there any Japanese reason to prefer one way of thinking over another?

Another question, or maybe the same question? When writing kanji, Japanese will sometimes name the radical that they are writing. "Nimben" is one I hear a lot, which I refer to personally as "Mr. T". Could it be that there is a "pepper-plant" radical that is clearly different than "stand" on top of "tree"?

[ファブリス]

"spicy" is listed as "karai" in the Kodansha Kanji Learner Dictionary radical chart.

One website that can help dissecting characters is http://www.zhongwen.com.

It says "red pepper plant" is a phonetic :
"wood 木 punishing 辛"
Ancient name : "thornbush"

Could be an ancient character then, but now only a phonetic, so it's not listed as a radical on its own.

Could be that James Heisig also got the "red pepper plant" from the "thornbush" reference. Many of his primitives are hints to the chinese radicals.