The United States of America is a Federation of States, as it's name implies. It is a good model, and something which has inspired some of our planned reforms. However, on the issue of secession, being a Federation, the individual States should at all times maintain the sovereignty to be able to decide to secede or stay in the Federation. It is a voluntary federation of States which have united to fight the British, and then expanded as the Americans moved West and South. Therefore, we defend that the right to secede from the Union is a fundamental right of the states that are in the United States of America. The people of the States that chose to secede supported the decision, and therefore the United States should respect their wish and sign a peace declaration that recognizes the right of individual States to secede.
However, Spain is not a Federation of States. Spain is one State, one government, one country. To acknowledge the right of a minority to secede from Spain and create their own country is a dangerous precedent. Taking the example of the United States, it would be the equivalent to one third of Pennsylvania deciding that they should be independent, because they don't want to pay taxes and provide soldiers to the Federal Government. Furthermore, in both the cases in Spain, the independence movements aren't representative of the population. They are radical movements, supported by foreign powers, in yet another assault on Spanish territorial integrity by France, that attempt to manipulate the population by the most despicable means in order to be rewarded by their foreign masters. We believe that the above also makes it entirely different from the situation of the Confederates.
Lastly, you could argue about the German federation and their right to secede, and I do agree that, if the people shows support to secede from the Federation, they should be allowed to do so. However, the reality is quite different. People yearn for Unification. They know that they will continue to be under siege from powerful hostile neighbors, such as the despicable France, the large Russian Empire, and the Austrians, and that only united can they break the siege and be treated amongst the nations of Europe as equals.