Seat Selection on code share flights

[KLOS]

If I ( non gold/silver) book BA to Tampa via Miami in WT I can't select my seat at booking on the first sector but can on the AA leg to Tampa. If I were to book directly through AA ( or USAIR or Iberia) does this mean I could select at booking or are code share passengers on the partner carriers bound by the BA charge at booking rule. If they are not it would render the BA policy pointless. The same re a BA direct code share to Tampa

Thanks