It may come from the fact that the fast WAN port adapter you can use is a 155Mb POS or ATM inteface, the 7200 does not support the 622Mb interfaces.
You can, however, use the PA-GE which gives you a 1 Gigabit Ethernet interface which runs at approx 400Mb or use the NPE-G1 whci gives you 3 full speed Gigabit Ethernet interfaces
