Why is the A330/A340's angle-of-attack protection disabled in alternate law, even if the AoA vanes are operating normally?
1. AoA vanes are speed sensitive

An AoA vane is in essence a weather vane, but due to various aerodynamic interactions it's not perfect. From flight testing they would have worked out how speed affects the position, and based on that the sensed position is corrected by speed input.

From the A330/A340 FCTM (Flight Crew Training Manual) section 8.110.4:


  The ADRs provide a number of outputs to many systems and a blockage of the pitot and/or static systems may also lead to the following:
  
  
  (...)
  Alpha floor activation (because AOA outputs from the sensors are corrected by speed inputs)
  (...)
  


2. Alpha floor inhibition

The above suggests alpha floor could have been activated due to the blockage, but unlike the A320, alpha floor is inhibited on the A340 above Mach 0.53 (no reason is given). (Source: A340 FCOM 1.22.30)



3. Crickets + stall

That stall warning in alternate law is based on a computed VSW (V Stall Wanring), and not the AoA. (A340 FCOM 1.27.30)



The A340 FCOM and FCTM are obscure as to what goes into the VSW calculation, but one of the inputs for that speed for the A320 is the calculated gross weight.