One reason for this is possibly the plethora of content available online and via BBC Sounds. The BBC might find using it's own 'skill' to access this content will make indexing the collections easier such as
'Beeb, play Podcast Desert Island Discs from 24th July 1995' or
'Beeb, play The Latest Desert Island Discs'
should be relatively easy to control the massive library using specific Beeb commands that might not translate across generic available broadcaster skills.
Must say I feel like this is the idea, to provide a way of accessing BBC content. If you're British, there's something uniquely attractive about content from the BBC, so while they say this will be available across many products such as iPlayer it makes sense to be able to ask third party devices for beeb content only.