Depends if you want to condone IP theft (compatible independent developments is of course different but I'm not sure this is the case). R&D, Support, good documentation in English and accuracy of specs come at a price.
I don't think they do IP theft from STM32 (if it's even possible at current node size). They have very thorough datasheets different from STM32 ones and their own SDK with unique code (although it seems to be inspired by STM32 libraries, but absolutely not theft).