Before I ask this, I wish to emphasize it is not intended to ignite a firestorm, but because I'm genuinely curious:

1.  Why would one wish to disable the no e-mail option?

2.  What does it gain a group to do so?

I'm "no e-mail" virtually everywhere, except on a couple of very, very low volume groups where virtually any post is a "special notice" in my world.  I can't imagine there aren't a lot of folks who might join a group who will not do so if they must use the e-mail interface rather than being able to choose the web interface.

I know if I were forced to be e-mail only that would be a deal breaker for me because I don't want any e-mail traffic from my routine participation on Groups.io.  I can do it all via a web forums interface, which has been my preference for "this sort of thing" for decades now.
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Brian - Windows 10 Home, 64-Bit, Version 1809, Build 17763 

     I can hire one half of the working class to kill the other half.

           ~ Jay Gould, U.S. financier & railroad robber baron (1836 - 1892)