However, the rules also prevent procurement officers from responding to collusion by, for example, raising reserve prices, or by promising one of the bidders a big share of the future procurement if she offers a better price.
This trade-off is examined theoretically, in a recent paper by some middling economists entitled Horizontal Mergers in Optimal Auctions, where procurement agents have the flexibility to design auction rules in response to the threat of collusion:
Merger [or collusion] effects are ... much smaller than in open auctions because bidders compete more against an optimal mechanism than they do against each other. As a consequence, there is less competition for mergers [or collusion] to eliminate.
This implies that collusion would have much smaller effects, if the auctioneer had the ability to react to collusion, e.g., with a higher reserve. But giving the auctioneer this ability could also lead to patronage (corruption).