Its not like they are looking for funeral costs covered and the family are appealing for funds. In that case absolutely, those clubs would be expected to offer the money.
The game is being organised by a collection of Millers friends and former teammates. They want to have the game in Cork, as it is Liams home county. Not Croker, Old Trafford, The Aviva or Celtic Park. Surely you understand the sentiment behind that?
Now, in Cork there is PUC, or the other option which is Turners Cross. Turners Cross holds 7,000. The game being played here means that alot of people who want to go to support the event will not get tickets. Some may be going to see the big names playing (which is why they got famous ex players involved) and some are going to pay tribute to the mans life as a sportsman and as one of their own. Regardless the reason, more people at the event means more funds raised and would also add alot more to the atmosphere and to the occasion for Millers family. The GAA are in a position to provide all of those things to the event and wont its as simple as that. The FAI and his former clubs are not in that position to offer that.
Liam Miller was an active playing member of his local GAA club too.