As a long-time rPi user and embedded developer, it’s really not about ideological purity (or at least not entirely). There are real ramifications for the lack of open documentation for the GPU interfaces on the Raspberry Pi that make it really hard to use (as in utilize) if not running a mainstream Linux w/ binary blobs and device overlays. The GPU even does the system startup and initial boot loading; it’s not just closed source, it’s also entirely opaque and undocumented (documentation only available for bulk purchasers from Broadcom and under NDA, purchasing Raspberry Pi in bulk doesn’t get you the SoC documentation).

(Unlike typical devices, the CPU doesn’t do system startup and initialize the GPU as an add-on or co-processor, on the Raspberry Pi the situation is more or less reversed.)