[madars]

Looks like & saves the day by invoking the command in a subshell and then in that subshell exec replaces the subshell with the xinput command. E.g. you can test it by

    $ cat ./a.sh 
    #!/bin/sh
    exec /bin/echo foo &
    exec /bin/echo bar &

which outputs foo and bar. Seems like a special case, just like exec without a command for file descriptor redirection.