I would think it would be easier to just dump the whole transponder and demux later on demand -- one (fat) stream to write instead of 4 for the same net effect.
As for why you can only then pull out the "Big 4" initially, that's probably a matter of licensing (the big 4 agreed to it) and consistency (different regions will have a different mix of channels on the relevant transponder, but they'll all have at least their big 4. Much easier to market "records all 4" than saying "records 4 in some markets, 5 or 6 in others...")
As for why you can only then pull out the "Big 4" initially, that's probably a matter of licensing (the big 4 agreed to it) and consistency (different regions will have a different mix of channels on the relevant transponder, but they'll all have at least their big 4. Much easier to market "records all 4" than saying "records 4 in some markets, 5 or 6 in others...")