I wouldn't call it "proper English" (although I often do) so much as "neutral English." There is one "central" form of the language that, in theory, everyone who calls themselves an English speaker shares. No matter what your local dialect might be, in theory everyone understands that neutral version, so they can communicate in it across different socio-economic, geographical or circumstance-dependent groups. If that central version is lost in favour of always talking in some pidgin dialect no matter the circumstances, then the universal ability to communicate in "English" is lost with it.
English does have a wonderful ability to incorporate words from other languages, and not just the ones from which it was originally created. That ability is what has made English into a world-level language, one spoken by many as a second language across the world. Another factor in that universality is its pretty basic grammar rules, where a lot of the complexity of other languages (genders, cases, tenses) has been stripped away, leaving it easy for other people to learn.
The words that it does take from other languages, though, are almost always vocabulary - nouns, verbs, adjectives. You can make up any old bollocks as a verb or noun and make yourself understood because those are open classes. But they still have to fit into the existing grammatical and syntactical structures to be understood. Grammatical words are a closed class, where it is next to impossible to change them. Look at the trouble anyone has had trying to come with a simple gender-neutral pronoun (the current attempt is "yo"). It's when grammar is lost and confused that a language breaks down. You might understand the concepts in play, but if you don't know how they relate to each other, how do you proceed? And given that English is so basic in its grammatical rules, it's not that unreasonable people to expect to follow them, especially native speakers.
A lot of my personal antipathy towards bad grammar is just personal pickiness - if I was a different kind of person who wasn't such a stickler for specifics it probably wouldn't bother me as much as it does.
But does it not wind everybody else up as well, when you see a professionally done sign outside a shop that is mis-spelled or mis-punctuated? You're using this to promote your business, presumably, and if you're not spelling basic words correctly in your first point of contact with potential customers, what kind of impression do you expect them to take away of you? And not everyone has the excuse of being a non-native speaker.