They turn iMessage on by default when you sign in because signing into your account and signing into iCloud purposefully conflated. Its very sneaky and arguably just as bad as the whole Microsoft local accounts lol. Also iCloud backups didn't use to be e2ee and iMessage backups which also weren't e2ee because the iCloud backups werent e2ee and held the keys to all the iMessage. Funny stuff.
To sign into your account locally and avoid iCloud, you have to sign in in the AppStore on both iOS and MacOS. Its fhe only way. On Mac, iCloud is activated by signing in the Settings place (same as iOS) or that Apple Media Account/Services modal they pester you with constantly.
This is to say, they turn it on by default under the facade that every Appl user would want to have all their private communications redirected through their servers via iMessage (its free and blue message so cool) and then there's multiple backdoors to your conversations even tho its said its e2ee and only accessible to you
Part of agreement
C'mon, seriously, nobody reads those. They're way too long and they get updated all the time. Its easier to disabke iCloud and use real proper apps like Signal. I avoid the Appl native apps like the plague. Their privacy nutrition labels should all be No Data Collected or only like Disgnostic stuff but basically every one captures UserContent and other invasive data.