I think I found the answer to my question
In the USA, cryptocurrencies are considered commodities because there is futures trading in cryptocurrencies.
I can't follow this justification, but the point is, in the USA cryptos - not only commodity-backed cryptos, but all cryptos - are considered commodities.
Thank you to forum member CebuBitcoin for having pointed to this article
US Judge Rules to Bring Alleged Cryptocurrency Scam Under CFTC Oversight
A U.S. federal district judge has ruled that an allegedly fraudulent crypto token meets the definition of a commodity, bringing the case under regulators’ purview, Finance Feeds reports September 27.
Judge Rya W. Zobel of the Massachusetts District Court ruled Sept. 25 against a motion to dismiss a case that had been launched by the Commodity Futures Trading Commission (CFTC) against an allegedly fraudulent crypto scheme known as “My Big Coin Pay Inc.,” reported to have been based in the state of Nevada.
Readmore: https://cointelegraph.com/news/us-judge-rules-to-bring-alleged-cryptocurrency-scam-under-cftc-oversight
The salient passage is this:
“Here, the amended complaint alleges that My Big Coin is a virtual currency and it is undisputed that there is futures trading in virtual currencies (specifically involving Bitcoin). That is sufficient, especially at the pleading stage, for plaintiff to allege that My Big Coin is a ‘commodity’ under the Act.”