In Vodafone the 2nd largest UK mobile network carrier (in terms of revenue) does indeed modify the page to include the failing JavaScript and also does respect the no-transform header.
Actually a Vodafone employee wrote the initial draft and was an editor for the W3C Content Transformation Landscape Working Group Note in 2007 which outlines Content Transformation proxies handling things like this.
And obviously the HTTP spec mandates how a proxy server MUST handle the no-transform header.
Sadly this is not “ethically bizarre” this is “normal” and within spec except for where a no-transform header is included, then a proxy “MUST NOT” transform the data.