Well I'm sure they'd love to offer a 5k upload cap if they could. There is a minimum upload that you have to have to be able to download at a certian speed.

It's the "laws of bandwidth". You have to have a certian amount of upload in order to have certian amount of download.

Example (I'm not using real numbers). I you want to download at 2mB/sec, you need to have at least a 200kB/sec upload. If you were to increase the download to 3mB/sec, it wouldn't work unless the upload is also increased via the d/u ratio.