There is definitely no direct evidence, either historically or in the film, that those who defended the rights of the kidnapped Africans in the Amistad case acted out of political self-interest. Those who defended the Africans were the leading abolitionists, Theodore Joadson, a former slave, and Lewis Tappan who enlisted the help of Roger Sherman Baldwin, an attorney specializing in property law. While Joadson and Tappan both first sought assistance from former President John Quincy Adams, he at first refused, saying that he neither supported nor condoned slavery; however, he eventually joined the fight to defend the kidnapped Africans, and his defensive speech before the Supreme Court was quintessential in convincing the Supreme Court to rule in favor of the Africans. But why did Adams have such a sudden change of heart? Was his change of heart based solely on political self-interest, or did he have more noble reasons?
It's important to note the case hinged on the question of property rights. Since La Amistad was a Spanish schooner, Spain's queen Isabella II claimed the slaves as Spain's property and wanted monetary compensation from the U.S. for impounding the ship and slaves; however, since the Amistad was also taken into custody by a US ship, according to the laws of the high seas, the captain of the American ship was entitled to salvage rights, making the ship and slaves rightfully the property of the US as well. It should also be noted that Spain lied about the slaves, denying they had broken British, US, and even Spanish law forbidding international slave trade, saying that the slaves had been born on a plantation in Cuba and were therefore legal domestic slaves. Hence, the case brought out the questions, did the slaves have the legal right to mutiny on the Amistad, and under the laws of the times, did anyone have the legal right to own the slaves? As it became apparent that Spain had broken their own international slave trade law by kidnapping these slaves out of Africa and then trying to transport them to Cuba on the Amistad, it also became apparent to Adams that the trial brought up questions about justice. Adams especially had a change of heart when he heard Cinque's story of brutal kidnapping translated from his African language, because it was at the point Cinque's story was exposed that it also became apparent laws were broken by the Spanish. More importantly, if laws were broken by the Spanish, then the slaves cannot lawfully be claimed as anyone's property. Hence, it can't really be said that Adams had political self-interest in mind when he changed his mind in defense of the Africans. Instead, even though he did not yet condemn slavery, he valued justice, and as a defender of the Constitution, he valued personal rights as well as the right to own property. Hence, in deciding to defend the Africans once he learned they had been illegally stolen, he was defending his own sense of justice as well as the principles he valued that are protected in the Constitution.
As for the abolitionists, abolitionism was a movement fueled by the religious understandings of English Quakers and evangelicals, as well as by moralist thinkers from the Enlightenment. Those who moved to put an end to slavery all did so because they either understood slavery to be unchristian or to contradict Enlightenment philosophies. Lewis Tappan especially became a Unitarian, a church that advocated peace and therefore also for the abolition of slavery. Hence, even for the abolitionists who defended the Africans, we cannot say they did so based solely on political self-interest but rather on moral and religious principles.