Similar but not the same. You are obligated by law to accept US dollar bills (any denomination) for the satisfaction of debt. Whereas you're not obligated to do so in the case of these non-voting shares. In a certain sense, by buying these shares, you are assuming there will exist other people who want them too (a market) whereas US securities guarantee a market (300 mil + US citizens).
It is true that the US guarantees liquidity for US dollar bills, but liquidity is hardly an issue for a public stock large enough to be listed on the S&P 500...
You are correct of course. My intention was to point out that this is a great step so that the S&P 500 remains that way. That you don't let in companies offering such shares in the same index (and thus market) as ones that don't.
Of course the ones that are already listed can continue to do so. But still a good step in the right direction IMO.
