Nifft said:Pages 55 (last paragraph), continuing into page 56.
Oh, good catch. So I think you have to read very tightly: "proficiency bonus... from your weapon" and "associated keyword". From this tight reading, the only possible bonuses are 1) from a feat, proficiency (if there's a weapon tag) and 2) from the item itself, enhancement (if there's a weapon or implement tag).
This pushes the problem somewhere else. For example, the feat sweeping flail, p206, is neither 1) nor 2). So possibly it doesn't give a bonus to attack (absurd reading of p56). More likely, p56 doesn't list all the possibilities.
I agree with you, I don't think you should gain the prof bonus, but I think the easiest fix is to write that in the paragon path.