The good news is it is executing the command in the first place and no errors are being generated. The bad news is it is treating “on 0 | cec-client -s -d 1” as the string to echo. In other words it is not piping the output but treating the whole thing as one big string to echo.
One thing to remember when using the Exec binding, it does not behave the same nor does it have the same capabilities as when running in the shell.
In this case I would try the following for the command to see if it works:
"echo 'on 0' | cec-client -s -d 1"
What was the output when you ran the shell script?