In US all cryptocurrencies are treated as commodities and regulated by Commodity Futures Trading Commission under the U.S. Commodity Exchange Act but when Ethereum switches to Proof of Stake they will be regulated as security by Securities and Exchange Commission (SEC) and all "stake" holders will need to register their stakes and comply with the laws that govern the Securities Industry[1].

Under the US law "security" is "The term “security” means any note, stock, treasury stock, security future, security-based swap, bond, debenture, evidence of indebtedness, certificate of interest or participation in any profit-sharing agreement etc."[2]

Ethereum falls under this since it is "profit-sharing agreement", it is an investment(stake) which allows other investors(stakeholders) to profit off of that owned security.

Proof of stake cryptocurrency allows stakeholders to profit by holding their coins(stake) and if they do not comply with all aforementioned security laws(reporting, market controls etc.) all their coins(stake) can be seized and Etherum can be shut down by SEC as an unregistered security.

[1] https://www.investor.gov/introduction-investing/investing-ba...

[2] https://uscode.house.gov/view.xhtml?req=(title:15%20section:...