Unfortunately, the popup Google chose to use looks like an advertisement. The red "new" banner and animated gif.
They might have reacted differently to an actual dialog at first click that says "This will inject an image into your outbound email, and mute the conversation. Continue?" (with the default choice set to "NO".
It would be cool if websites didn't pop up dialogs for stupid reasons. 90% of dialogs that appear without user interaction are intrusive and unnecessary so users get used to closing them. Would you like to take a survey?, download our mobile app?, full screen ads, and my favorite - "our site has changed dramatically, here's a tutorial."
They might have reacted differently to an actual dialog at first click that says "This will inject an image into your outbound email, and mute the conversation. Continue?" (with the default choice set to "NO".