They had to do that as both Sam Alito and Clarence Thomas were taken blatant and obvious bribes beyond anything the SC had treated as acceptable prior and thus they had to create a law that basically says 'nothing is a bribe unless you can prove it changed what the person would have done had they not taken the bribe'.
You have to now get in a person's mind and prove their intent changed, an impossibility within law or you have to catch them saying "ok i changed my vote... due to that money". As long as they never say that part out loud, a guilty verdict is near impossible.