Napoli released a statement saying the National Court of Appeal missed a chance to "give credibility" to the system despite reducing the length of Gonzalo Higuain's ban.
Higuain was initially given a four-match suspension for his red card and subsequent furious display of dissent during the Serie A side's 3-1 defeat at Udinese on April 3.
Napoli were able to get the ban reduced to three games after an appeal, but the Italian club were still furious with the decision to punish the Argentine.
"We take note of the decision and see that it lacked the necessary courage to really shed light on this and do justice to the player," a Napoli statement read.
"This is a missed opportunity to give credibility to the entire system.
"We are extremely disappointed."
Higuain, 28, is the Serie A top scorer this season with 30 goals and will make his return against third-placed Roma on April 25.