Thanks for the link to the discussion thread.
My leaning is more for flavour reasons. Even at my table top game magic items are rarely rarely found in shops, and then there is a selection that matches the flavour of the shop. They are treasure hoard rewards for me. My players are all historically non-gamers, so I pick and choose what would be cool items for them. Then again, I think magic items are one of the weakest elements of 4e, so when PCs get treasure they tend to be cooler and less limited than 'legal' 4e treasure items.
But for ease of workload I am happy to fulfil players wishlists as per the norms of L4W and allow whole scale buying of desired treasure as in this interim adventure situation. Saves me time and energy. If you want to RP how you came about such treasure of course, well that would be just lovely (but not necessary if that is uninspiring to you).
I think I am good with limiting boons to non-purchasable, in-game rewards that make sense in the context of the adventure. That way there is no abuse, but the restriction is not exactly to one boon; and receiving them replaces receiving another magic item so all PCs are at similar treasure levels.