My question is not about how to get it, I know it can be done by copying the key from ubuntu-user's folder to root's one.
My question is about to understand the motivation after this by default-limitation.
Every article, post, tutorial, etc. I have read explains that it is configured this way due to security issues, to improve it, but since EC2s, at the same time, recommends not to assign a password to the root user, any person who access to the system as ubuntu user has immediate access as root, so I don't understand this motivation.
I have thought that it could also be because many attacks may use root as default user, so it can be a good idea to use another user less common, but again, since the default user is ubuntu in each and every EC2 (the ones with Ubuntu, obviously), this doesn't seem to be a great motive to do it this way either.
Finally, I guess it may be more secure to use ubuntu user instead of directly root just as a "personal firewall", I mean, as long as one has to make a "sudo -i" to become root, one may avoid making some mistakes just because during those 2 seconds one could have the chance to think "Wait, this command I was going to run might not be such a good idea...", and that point I do understand it, but it is the only one which really seem real to me.
Summarizing, my question is whether this last point is the only one I should be worried about in case I allow root login via SSH or I am missing any important/real security point when allowing SSH root login?