Thought I posted this but Fusion is taking a look at doing just so, also StorJ does the same thing shards are encrypted before being sent to the Network so in technicallity shards can be encrypted even though StorJ does not do this using Private Keys.
AFAIK in the case of StorJ the encrypted content is sharded, not the private keys themselves.
Come to think of it, let's check the whitepaper:
Files should be encrypted client-side before being sharded. The reference implementation uses AES256-CTR, but convergent encryption or any other desirable system could be implemented. This protects the content of the data from the storage provider, or farmer, housing the data. The data owner retains complete control over the encryption key, and thus over access to the data.
So yes, the private key stays with the content owner and should not touch the network. The content is encrypted before being published on the network, after which the network takes care of sharding the (encrypted) data.