They're just following Apple's lead.

When iPhone shipped, any time you emailed a photo it would append "Sent from my iPhone" to try to virally market. Same or at least similar slimey tactics

There was at least a fig leaf of “I’m typing on a mobile keyboard” excuse my typos”.

That’s just an email signature, configurable (or removable, if you prefer) by the user. This is completely different, in my opinion.