I thought the GPL asked you / forced you to host the files, which is quite different from just supporting it.
Isn't this what the following bug was about? http://bugs.archlinux.org/task/5355
From the GPLv2
3. You may copy and distribute the Program (or a work based on it, under Section 2) in object code or executable form under the terms of Sections 1 and 2 above provided that you also do one of the following:
a) Accompany it with the complete corresponding machine-readable source code, which must be distributed under the terms of Sections 1 and 2 above on a medium customarily used for software interchange; or,
b) Accompany it with a written offer, valid for at least three years, to give any third party, for a charge no more than your cost of physically performing source distribution, a complete machine-readable copy of the corresponding source code, to be distributed under the terms of Sections 1 and 2 above on a medium customarily used for software interchange; or,
c) Accompany it with the information you received as to the offer to distribute corresponding source code. (This alternative is allowed only for noncommercial distribution and only if you received the program in object code or executable form with such an offer, in accord with Subsection b above.)
My interpretation of b is that you don't actually have to host the files but you must be able to supply the sources upon request. As the PKGBUILD is readily available and is able to retrieve the source files, that condition is met.
Hosting all the source code for every package would be inhibitive.