Shipping to APO/FPO addresses?

I am baffled on why alot of sellers on EBay and alot of companies will not send merchandise to APO/FPO addresses. With all the military is doing for them.....it's obvious. A few have told me it's too much trouble, or more expensive......is it really more expensive?? I have never heard that. Does anyone know if there is website keeping track of sellers/companies who are not on the militarys side and refuse to do this? Please let me know any good information you might have. Thanks.