That is a design decision made by Telegram. My e2ee Signal and Matrix groups sync just fine across devices, preserve message history, allow message forwarding, etc.
Not enabeling those features is a nudge against using the e2ee-feature. Facebook Messenger does the same by crippling their encrypted chat experience.
The reason most people on WhatsApp enable unencrypted cloud backups is not because they really desire their message history to be leaked to Google/Apple but because they get occasionally nudged by a popup to enable it.
Those nudges work. It does not matter whether it's a good or a bad action they nudge to make one central assumption about them: settings should not be interpreted as user choice if the results aren't tested against complementary nudges.