Odds are they have Resist Fingerprinting turned on. When I use it in a Firefox profile I encounter this all over the place. Drupal, FedEx.. some sites handle it better than others. Some it's a hard block with a single terse error. Some it is a challenge which gets blocked due to using remote javascript. Some it's a local challenge you can get past. But it has definitely been getting worse. Fingerprinting is being normalised, and the excuse of "bot protection" (bots can make unique fingerprints too, though) means that it can now be used maliciously (or by ad networks like google, same diff) as a standard feature.