Companies who do this get neither the benefits of being fully proprietary (full asset value of "IP" on your books etc), nor the benefits of being fully #FreeCode (community goodwill, volunteer contributions etc). Why do they keep doing it? Probably for the same reason other projects are using proprietary #SourceAvailable licenses. Because #VentureCapitalists think its a good idea.
@winniehell I'm not sure what size Mistpark is, but what about #NextCloud, #Loomio, or #JMP? There's also companies like #RedHat, which are technically support contractors not software creators, but they make a lot of upstream contributions to the software they use. Conversely, here's an example of a startup that failed to fund their team using a proprietary model and decided to free their code (although it didn't save their business): http://blog.buckybox.com/2018/07/reduced-service-announcement/