Keychain is opt-in. If you have it configured on your iPhone, it's because you accepted it when configuring your iPhone for the first time. You had to choice to refuse it - and you still have the possibility to disable it in your iPhone settings.
It's unclear how it works, I received a prompt in Firefox to save my password, and I thought that it was the Firefox browser that would be saving my password however, it turned out to be keychain. Now the keychain message pops on all the time, I will have a look in the settings to see how to disable it.