Liberalism is many time seen as an attitude to compromise or take a wishy washy approach. That may not be the dictionary definition, but it is how it is seen by some. In this case my guess is that those involved had no real patriotism in them and so they did what came natural, do something liberal.
It would be no different if there was a class on different religions and a group of Christians did a pledge to satan or some cult even if they did not mean it. In my opinion saying these people are liberal is a complement because in my opinion they are traitors.
Why is it wrong to not be Patriotic? How is that liberal? The lack of patriotism or the expression of patriotism is not a measure of liberalism. It merely is a matter of personal preference. Being patriotic is not a litmus to any conservative or liberal organization.
BTW, I have several friends who have a dual citizenship. They have allegiances to two countries. Is that treason? The United States Government allows this and has for almost all their existence. In fact, you can be a citizen of another country but if you agree to defend and protect the United States, then we will allow you to serve in our military. I served with several who were serving but citizens of another country. This practice dates back to the Revolutionary War.
For the record, the Constitution defines treason: "Treason against the United States, shall consist only in levying War against them, or in adhering to their Enemies, giving them Aid and Comfort. No Person shall be convicted of Treason unless on the Testimony of two Witnesses to the same overt Act, or on Confession in open Court. The Congress shall have Power to declare the Punishment of Treason, but no Attainder of Treason shall work Corruption of Blood, or Forfeiture except during the Life of the Person attainted."
Mexico is not an enemy of the United States, rather we are friends with Mexico and have many pacts of friendship with this country.
This is not treason. The Constitution never says you have to be patriotic; it never says you have to be in love with the U.S. Rather, it says that treason is only levying war, adhering to their enemies, and giving aid and comfort. In the United States, you have right not NOT love your country or to love another country. That is neither liberal or conservative, it is merely reality.
This is not treason: this is not liberalism. Rather, this was a study in Mexican civics. The teacher performed the lesson poorly, but it is not liberalism no more than a Mexican English class having their students say our pledge.