I struggle to understand how a TiVo-ized device running a mixture of GPL2 and proprietary code isn't a license violation. The code is distributed to the end use

I struggle to understand how a TiVo-ized device running a mixture of GPL2 and proprietary code isn't a license violation. The code is distributed to the end user as a single uninspectable blob, i.e. the firmware image. Yes, "under the hood", there are separate binaries - but this is purely an accounting fiction that holds no relevance for the user. Is any combination of GPL and non-free code allowed, so long as it emulates a Unix OS at some level?