I only stopped there for transit on a flight to Mexico. Just before boarding my flight I was told that I need a visa for the US, which is extremely weird because normally airports have transit zones where you don’t need any visa. But apparently the US is special, so you actually have to enter the country before going right back into the airport. This nonsense made me miss my flight.

Not like that, they didn’t let me board my flight from Europe to Texas. Even though I had a connecting flight to Mexico few hours later.

I am no longer on your side. if your journey is :

European country X -> Somewhere in the US -> Texas -> Mexico

Then of course you're going through security controls, etc etc. You're going from the international processing and flights part of the airport to the domestic flights part of the airport, which is the zone where any asshole in that country can travel in, so you're properly entering the US. Of course you need proper clearance and visas and all that.

I initially assumed your journey was

Non US country-> Somewhere in the US -> Mexico

which you'd be completely right