Can my employer dictate how I spend my time off?

I run a demolition derby every year. In 2007, I missed 3 days because I cracked a rib. Last year, I was injured, but missed no work. Recently, I had unexplained shoulder pain, and I ended up having to have surgery to repair it. During surgery, they discovered a previous injury consistent with an impact. Last year's derby was the only time this could have happened. Consequently, I'm out for 6-8 weeks while I heal after surgery. My employer recently contacted me and told me that since the derby injuries were causing me to lose time at work, if I ran any more derbies, I would be fired. Can they do this? How is this any different than injuring yourself by other means or being out due to an illness?