What is "proper punctuation"? Isn't it context dependent?
Not every instance of written language is written in complete sentences.
A sign that says "SALE" is normal, but a sign that says "Sale." would be unusual, maybe some kind of marketing or design choice.
Social convention around IMs and chat rooms in the early versions of live chat, in the 90's, capitalization and punctuation were not ordinarily used. Multiple sentences per message were also not the norm.
Text messages have always been somewhere between 90's style IMs (uncapitalized and unpunctuated phrases, not full sentences) and a full email message (full salutations and signatures). The convention depends on the context, and autocorrect has changed what is or isn't normal.
So a text message response that says "that's fine" conveys a distinct message from one that says "That's fine."
That's how human communication works. Trying to start every text message with "Dear Jake," and ending it with "Sincerely, Raymond Holt" would be weird.
ACK