Except that most firewall/router boxes like Linksys, NetGear and D-link don't have any QoS capability. A protocol, port, IP addr or URL is either on or off, possibly by shecdule. 99.992% of all home users aren't concerned about it.
QoS takes a little more CPU power and memory than these little devices generally have. Maybe future products might have a user-friendly switch in the GUI to "give web access better performance than downloads", but only if there is customer demand or a competitor offers it.
For anyone interested, here's a simulator for the web interface for one of these devices... (>o|-<
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(He said I could say that!)