Let's put it this way. You like pork, and aren't too huge of a fan of beef. Now, in your life, every day you are bombarded with the negatives about beef, positives about pork, and everyone who eats beef is shunned. It's all pork 24/7. One day you get kind of sick of pork, you know, since you have to eat it for every meal, even though there are a bunch of different preparations. Your friends and family look at you, aghast, for expressing interest in trying some beef. Beef-eaters are horrible people, you've been told, and none of their dishes are served anywhere near you. Okay, fair point, maybe you were wrong. You continue eating pork all the time, kind of wondering why some people eat beef, and what makes them really bad? You start to question the fact that they are really that bad. I mean, you kinda wanna try it, at least to break up the monotony of the constant SPAM you're getting. Eventually you meet someone who's willing to have some beef with you, something weigh just a little beef in it, just some beef boullion. A beef moderate, if you will. All your pork friends condemn her the same as if she were eating full-blown beef stew. I mean, how can't they? She's a bloody beef-eater! But you now have a taste for beef. Your social circle is absolutely disgusted with you, how could you betray pork by having a single meal of beef, you're an absolute abomination and deserve to be indiscriminately punched on the street. This rustles your jimmies to the point where now you want to explore beef-based culinary options out of pure spite. You find the beef-eaters are extremely accepting, and there are some who eat pork and beef side-by-side! Burgers and hot dogs, together, living in harmony. This was not the world portrayed to you by TV, radio, reddit, your friends, professors, etc... Beef eaters aren't all evil... So sometimes when you see pork propaganda, even if you are quiet most of the time, you post some recipes for steak. The porksters are furious, but that makes it all the more delicious.
That's not a general endorsement of a physical barrier on the border. It's an endorsement specifically of 650 miles of fencing in select areas, which by the way turned out to be an embarrassing waste of money that hurts local wildlife and landowners more than illegal immigrants. Just because people supported that doesn't mean they would support the gigantic boondoggle of building 2,000 miles of concrete over land and water terrain.