Here's a post where someone else was wondering about it with some suggestions:
http://www.redearslider.com/forum/viewt ... light=burn
Read down towards the bottom of the thread. Suggestions are to angle the heater so it's not as easy to rest on, put it in a pvc pipe, or get a "heater cover" for it. The heater in this case was, I believe, glass, unlike yours which is, I believe, black plastic? Have you felt the heater - is it hot enough to your touch that you think it could burn?
I haven't seen any instances on here where a turtle has been burned by a heater. I've seen a couple of instances where glass heaters have shattered, but that's all. I've got the same brand heater as yours and don't worry about it, thinking that if it got uncomfortable for Spot he'd leave it alone.
You need to be sure the heated part is always submerged. If some of it is out of the water it will get really hot.