First of all, she isn't transgender.  She is a biological female who has naturally high testosterone levels.   
  
Secondly, this is discrimination because the rules only apply to races between 400m and 1 mile, which are exactly the events she competes in. Why wouldn't the rules also apply to the 100m or 200m races, or longer events? Why don't the rules apply to other sports?  
  
There is only one answer:  Because the rules aren't targeting the issue, so much as they are targeting *her* specifically. That is discrimination.