在这种情况下，HTTP 400 似乎不是用来表示请求语法中的错误，而是用于表示此处描述的逻辑错误:HTTP 400(错误请求)用于逻辑错误，而不是格式错误的请求语法.

It seems that in this case the HTTP 400 is being used not to signify an error in the request syntax, but a logical error as described here: HTTP 400 (bad request) for logical error, not malformed request syntax.