> unless they're writing life saving software or ... you're forced to use them even if you don't want, the technology they decide to use is entirely their choice
The technology is always (or almost always?) their choice. However, if it is a public sector organisation, it owes it to the public to care about them, to be informed about devices they are using and the conditions they are using them in; and to strive to provide its services in the most accessible way. "How fast does a page load", "how fast does the page perform on users' devices once it's loaded", and "how gracefully it degrades under unfavorable conditions" should be important parts of the conversation.
Businesses in the private sector have fewer responsibilities to the public, and are primarily controlled by market forces; but still, it would probably benefit them to have similar considerations.