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I think I can blame that one on Congress. The relevant statute is 18 USC §666(a)(1)(B). It has a mens rea component of "intending to be influenced or rewarded in connection with any business, transaction, or series of transactions of such organization, government, or agency involving any thing of value of $5,000 or more" when "an agent of an organization, or of a State, local, or Indian tribal government, or any agency thereof
[1] [...] corruptly solicits or demands for the benefit of any person, or accepts or agrees to accept, anything of value from any person".
Compare that to the anti-gratuity statute for (federal) public officials
[2], 18 USC §201(c)(1)(B): "Whoever [...] being a public official, former public official, or person selected to be a public official, otherwise than as provided by law for the proper discharge of official duty, directly or indirectly demands, seeks, receives, accepts, or agrees to receive or accept anything of value personally for or because of any official act performed or to be performed by such official or person".
Merely accepting a thing of value given to you for or because of an official act (i.e. a gratuity) isn't proof of mens rea in 18 USC §666(a)(1)(B). The prosecutor must prove that you intended to be influenced or rewarded, e.g. with a prior agreement. All in all, 18 USC §666(a)(1)(B) sounds more like an anti-bribery statute than an anti-gratuity statute, so I can see how the SCOTUS majority came to that opinion.
[1] Limited by 18 USC §666(b).
[2] As defined by 18 USC §201(a)(1).