The US doesn't block websites. Occasionally the FBI will seize a domain after a court order, but that's it.
French people were already forbidden from transacting on Polymarket and Kalshi, and the services themselves were forbidden to take any market orders. Nobody disagrees with the regulations.
But apparently the French government still thought it was so dangerous that they needed to block the websites completely to protect the French people from themselves. This is where the line is crossed. There wasn't even a court involved at all.
This happens all over Europe. The citizens give their governments the power to block things, or mass surveillance in the name of "safety" and then they just take everything and EU citizens are left wondering why.