Why do Americans add the word ‘already’ like that to the end of sentences? It’s not necessary (and it don’t be good grammar neither…)
The usage of already as a post-end adverb is growing and serves to codify a morphemic indication that the clause has happened and completed in the past tense. The usage is become general and as such the usage is result in semantic bleaching and cementing as a syntactic construction.
Evaluation of the formation as incorrect grammar demonstrates a naivety to linguistics and shows that you view languages in a set manner as unchanging, while in reality individuals from 100 years ago would likely find your use of English appalling and terribly incorrect.
Does this in turn mean that your understand and usage of English grammar is wrong? No, it demonstrates linguistic shift and that you need evaluate what criteria you use to determine incorrect grammar.
In the usage of “already” at the end of sentences, you are observing a language shift that is moving outside of your acceptable modification range, triggering an internal syntactic rejection. Rejecting language shift for personal use is fine, but personal rejection does not equate a complete value judgement, instead you should analyze the usage systematically and if the usage is not systematic and consistent in its meaning, THEN it is incorrect.
The fact that you generalize a group as using the construction clearly indicates that the usage, from your perspective at least, is system, and should lead you instead to accept it as a diverging language form that YOU most certainly will NOT be using.
Some people over use the construction to serve the purpose of hyperbole and that’s annoying, but admit it, language must adapt or die, much like the Panda. So let’s get it over with, let old English rules, and stick in the mud Pandas die out.