Although it may not be a popular opinion, here is the reason: there is only one Google (at least for social stuff) and one login system now. Google plus is the "Google account" you need to use any of their social services. I am frankly surprised that they allowed the login of their acquisitions to stay fragmented for such a long time.
The Google Account was already in use before Google+ was created. The difference was that you could have a mixture of services enabled for your account (I only use Gmail, I never use Picasa so I don't have it on my an account). Now they seem to force you to always have the Google+ enabled even if you never use it. Including for older accounts. I guess this is what makes people angry. It's like being prompted to creat…
Some social services like Youtube and Google+ require it, but you could still use Gmail without having Google+ enabled.