I am happy if someone claims that something politically isn't possible in the US. (Gun control may be a good example) I can't claim otherwise, my issue is when instead of arguing that, someone claims that a measure wouldn't have the same outcome in the US or technically isn't possible in the US without any evidence of such.

It shifts the blame from the current politics which is something people can engage with to "reasons out of anyone's control", which just helps maintain status quo.