AES works a block cipher with a block size of 128 bits. So if the decryption program knows the key and the IV, it can return plaintext from the first block -- there's no reason to decrypt an entire segment/fragment first.
I'm guessing the PlayReady implementations you see are only doing it on a file basis because that is slightly easier to implement. But there's no technical reason that a client would need to retrieve the entire file before initiating the decryption.
You can verify what a client is doing by making a fragment/segment extremely large and checking client behavior.
A quick search found me this project which is decrypting HLS as it reads: https://code.google.com/p/mlbtv-hls-nexdef/source/browse/trunk/mlb.c