Upvote:3
My recollection of history books is that the money was regarded as a bribe due to the money was going not to the French national coffers, but rather specifically to the French aristocrat/politician/diplomat Charles Maurice de Talleyrand-PΓ©rigord, often referred to as "Talleyrand".
Wikipedia has a sourced claim of the same (Stinchcombe, William (October 1977). "The Diplomacy of the WXYZ Affair". The William and Mary Quarterly. 34 (4): 590β617. JSTOR 2936184. pg. 598)
Also, there was (according to Wikipedia) a culture clash; such things were apparently common in continental European politics, but not among the Americans.