I don't know whether such extreme talk is legal, no, but it's quite common for lawyers to talk in this way.
Remember those letters you got when you forgot to pay for something on time? You know, the ones with a nice little addition saying "if you do not pay before X, we will be forced to take legal action."
It's the same thing, whether they will take action or not. The intent behind the statement is 'this will make it more likely that our victim is going to pay, so we don't have to deal with expensive legal stuff.'
Remember those letters you got when you forgot to pay for something on time? You know, the ones with a nice little addition saying "if you do not pay before X, we will be forced to take legal action."
It's the same thing, whether they will take action or not. The intent behind the statement is 'this will make it more likely that our victim is going to pay, so we don't have to deal with expensive legal stuff.'