The reason for that is due to HDCP/SOC, which are both copyright protection software's that are enabled when streaming with an HDMI cable. The software assumes that, since the Slingbox is a media device, that you're using the HDMI cable to download the program. You should also be able to adjust and use Component rather than Composite to stream, and then you'll still be getting the same high definition quality that you see with HDMI.
I hope this helps!!