Hm, personally I believe it's the server admin's responsibility to remove AFK players to spec (who've been AFK for some time of course) to avoid spawn killing / afk camping.
Sure it's xp whoring in a way, but if players remain in a team while being afk for a long time, so be it.
This is not of the same type as deploying the sh*t outta the spawn area with med and ammo packs
EDIT: ps: if the server allows spawnkilling, then it doesn't matter if players are AFK or not, spawnkillers aren't infringing any server rules