It is not polite/non-polite. It is more about standard literary language / spoken (vernacular) language. "Hrajou" used to be non-standard and was made standard relatively recently. In speech it is very common, even when speaking the standard form and not the common dialect. In the Common Czech dialect "hrají" is non-existent.
It is actually called "hovorový tvar". It is hard to translate it to English and it is a strange concept - an alternative form which is standard and still less standard than the absolute standard. A strange invention indeed.
It has NOTHING to do with politeness, only with formality.