Thats right, but what if some one finds our password and logs in from somewhere else . so he can log in easily and bother the main user.
That's a security issue and remedial actions would have to be taken e.g. user policies, password policies, tracking down the person with the "stolen password" and blocking his/her IP etc.
I think the first login (session) shouldnt be terminated at all by other sessions.
That is a usability issue and I highly doubt would be implemented, nor should it given the amount of devices a user could employ be it desktop, laptop, phone, tablet or even TV.
Normally it really annoys me when people post saying something shouldn't be done, ignoring the reality the person that posted asking for help has to live with, but in this case, changing it the way you want to is, IMHO, a very bad idea unless it's in a severely restricted case such as Moodle only accessible via LAN.