[plt-scheme] question on cond

I have a student working in the EOPL language level who submitted 
programs whose cond expressions do not contain the else keyword. It 
isn't replaced by #t; it just isn't there. Having drunk deeply of HtDP 
Kool-Aid, I was not aware that a cond clause could be of the form 
[test-expr]. But the PLT Scheme Reference, section 2.12, says that 
test-expr is not in tail position with respect to the cond form. Why 
not? --PR