Signal could modify the application so a remote flag in the Play store binaries could be triggered to exfiltrate data as well. But the key distinction is the normal path of Signal gives them absolutely nothing they can tell anyone other then the bits they've put in the disclosure reports (namely: date and time an account ID used Signal I believe).
At any rate, the discussion going on here is about how Durov has been arrested because Telegram refuses to respond to law enforcement requests, when they do have the ability to do so; and if they were to actually implement E2EE by default (and for group chats), Durov would likely not be in trouble, since Telegram would be unable to provide anything when requested.
I suspect that isn’t the motivation. GDPR says that you have to give users choices about data stored like this (including right to be forgotten, how it’s processed and used and so on), and this becomes a technical, legal and commercial nightmare very quickly. The easier route is just to get rid of it if you can.
This saves Google money (it likely wasn’t that useful to sell to advertisers), makes legal compliance a lot easier and de-risks them from very large fines.
I suspect that the EU lawmakers didn’t think about second order effects like making it harder for law enforcement to access this data in scenarios like this.
Should they have to comply with law enforcement they have much more straightforward ways of doing so than capturing messages off screen.
