The only reason you're opposing this, is because 1. It's Trump 2. You want an open border.
First of all, the border which is the demarcation point for a nations sovereign territory absolutely falls under the purview of the Government, anything to the contrary is laughable. The fact that LibDerp Nation is trying to say a border and border security wall falls into the category of Bill or Barry stroking a pen and grabbing hundreds of thousands of acres of land in Utah or Nevada is comical in every respect.
I do not care that it's Trump. The border is one of the issues I care about least either way. I'm opposed to an "open border," but I'm not sure a wall is the best allocation of resources. Maybe it is? Who am i to say? If Congress put money up for it, I don't care. If Congress didn't, I don't care. But honestly - wall/no wall. I don't give a crap. My beef has little to do with the wall itself.
My main beef with this is the president unilaterally expanding his power. This is why we have congress. This is why we have separation of powers. Letting the president unilaterally push through what amounts to legislation of his campaign promise tosses all that crap right in the garbage.
I griped about Obama doing similar stuff (just as I'm sure you did).