Mining pool has to decide which Merkle tree hash it distributes to the miners, with or without some transaction. They can not switch at will. If they send the hash without the wining pump-up bid, it will be easily noticed by many people since sooner or later they will solve a block and it will be shown they were broadcasting the transaction they themselves did not include in the tree.
If you are speaking about them not cheating, anyone can bid up his own auction, being mining pool or not. This could not and should not be stopped by the system, happens also in real life.