I believe it isn't in most Western countries. Life sentence is not only about the crime committed. It's to isolate individuals who just cannot be redeemed.
It's frequently not a life sentence in the US, and a "life sentence" in the US is actually not a life sentence, as people can get out on parole in under two decades (exact time varies by jurisdiction).
It is a "life sentence" really, as the offender is only released on parole and is on licence for the rest of their life, so can be required to abide by license conditions or be recalled to prison at any point.
(assuming US law is similar to the UK)
That's a semantic argument.
I think when most people hear "life sentence" they think the person will spend the rest of their life behind bars, so the phrase "life sentence" doesn't clearly communicate what actually happens.
Here in Canada I immediately think of this one murder back in 2008 a man killed another man on a greyhound, beheaded him then and then cannibalized him [1]. He only spend 8 years in jail then was released. He was deemed mentally not responsible for his crime and released.
[1] https://en.m.wikipedia.org/wiki/Killing_of_Tim_McLean
Read about Junko Furuta murder case, and you'll get even more mad.
I started reading and could not finish. This is a horrible story. It will hurt your mind for a long time to come. Please read with caution as the story is very graphic in the horrible things the girl suffered