He was awarded the first prize for being the fastest runner.

for是介词,介词后可以跟名词和动名词,不能跟动词原形.
the fastest runner意思是“跑得最快的选手”；runner是名词“赛跑选手”的意思而不是形容词,也就无所谓比较级；得第一（the first prize）的当然是跑得最快的了,所以用最高级fastest.