There's no hard rule about where national airspace ends and space begins.. a lot of times they fly too high for planes to reach, and under certain circumstances balloons are allowed through national airspace.. so if the balloon isn't causing a problem (like being a hazard to air traffic), people just ignore it... weather balloons fly through our airspace all the time.
But the US public saw this massive balloon and demanded it be shot down, even if it caused an international incident.. and luckily nothing serious happened. So now that the cat is out of the bag, we're going to shoot down every balloon that hasnt filed all of the required paperwork that enters out airspace that's reachable by our aircraft... because 1) the public demands it; 2) it'll become political fodder if the white house doesn't; and 3) the international issue it generates is easier for the white house to handle than the political criticism.
If China never sent that large balloon.. they could have sent a dozen more small ones and no one would have cared.. just like we never cared about the 3 during the previous administration.
Edit: bobbyjo pointed out this link: https://www.wsj.com/articles/chinese-balloon-carried-antenna...
Since that likely settles the balloons intent… we can add 4) now that we know it’s a spy balloon, we’re going to shoot down all the balloons. Before the large balloon was shot down, no one would have bothered to check. No one is stopping weather balloons at the border. Big mistake on chinas part.