I assume they do this for speed? When you are running that many videos you could save real money by making encryption cheaper and pack more onto each server. Is there any other justification for this?
Speed is not a good reason. While RC4 is great for weak chips, like microcontrollers and such, it is actually not that fast in modern hardware, needing at least 4 cycles per byte. The fastest combination of RC4 and MD5 in OpenSSL manages 6 cycles per byte [1].
With AES-NI, AES128-GCM runs at roughly 1 cycle per byte. Without AES-NI, the recent Chacha-Poly1305 ciphersuite Google has been experimenting also runs at between 1 and 2 cycles per byte.
