The presence of the apostrophe is an orthographic convention and has nothing to do with language in the least. There’s no apostrophe in it’s when people pronounce it.
Hmm, I'm intrigued by your assertion that orthography has nothing to do with language, but indeed you're right that this is an issue which relates to spoken language, so I'll ask a different question:
How would you describe the use of "isn't" where "am not" is generally considered grammatical?
For example "I isn't a liar" rather than "I am not a liar".
Again, you’re missing the point. I can’t prescriptively judge whether something is grammatical or not, nor can anyone else. Grammaticality isn’t something which someone decides — a grammatical sentence is by definition a sentence which speakers of the language produce and understand. It is grammatical if people use it. If there is a particular dialect of English where they use isn’t as the first person singular form of the copula, then it is grammatical.
It would not be grammatical in the standard American English dialect, which I think is what you’re getting at. But you have to be careful with your terms here, because in the technical sense ‘grammatical’ means an acceptable sentence of the language, whereas I have a feeling your understanding of ‘grammatical’ means ‘how we were taught to write in school to communicate to other people that we have been educated’. My entire point is that there’s a difference.
You would probably call the sentence
He been had that job.
completely ungrammatical. In Black Vernacular English, however, it’s perfectly acceptable. What’s more, depending on the pronunciation of been, this sentence can communicate a tense that doesn’t exist in standard American English. In SAE it can only be communicated by adjunct material like ‘for a long time’. Ostensibly, this is more efficient.
"Wrong" is the word I'd use, regardless of what definition you wish to give that word. What word or words would you use?