I don't think they force us to range weapon only. My interpreation is that FoK damage will be based off the ranged weapon no matter what type of weapon of equipped, however we will be able to poison our thrown weapon. This situation would point to FoK poisons only being able to proc if the weapon is a thrown and has a poison currently applied to it. This would lead to a situation where by we have a choice to apply three poisons for say crippling poison to slow the target with for a quick deadly throw using a previous example.
Ghostcrawler posted an update that seems to indicate that rogues will indeed go thrown weapon only:
We just want the ranged weapon to be more than a stat stick for rogues. Adding poisons to FoK is actually a pretty hefty buff. Yes, this means that bows and guns aren't of much interest to rogues (after leveling). But in this case we want Fan of KNIVES to be taken literally.
Seems odd that ranged weapon usage gets even more limited in scope, it's going to make for more loot table dilution. Bosses are going to have to drop an agi based thrown only for rogues, an agi based bow/gun/xbow only for hunters, and tank stat based ranged only for warriors.