The problem is that standard input is not always a pipe, and splice(2) requires at least one side to be a pipe.

$ ./test < test.c
splice: Invalid argument
(exit: 1)
$ cat test.c | ./test
$ ./test
(exit: 1)

In the first case, standard input is a file, which cannot be spliced. In the second case, it is a pipe. In the third case, it's a character device i.e. my terminal.