The entire justification for the H-1B visa system is so that we can bring in highly skilled labor to fill positions that cannot be filled with citizen talent for a reasonable price. If you're hiring H-1Bs at below market, your failure to fill the position is as likely due to your underpaying as it is an actual labor shortage. You cannot reasonably justify the need for H-1Bs if you're underpaying.

Further, if you use H-1Bs to fill positions that you could have filled with local talent, then you're taking that visa away from another company that might legitimately need it. Making H-1Bs dependent on bidding or pay seems a plausible way to ensure that they are going to companies with real need and not just companies that are trying to undercut the labor market.