By default, when the exec.Cmd is used, the child process is assigned the main proces group. As a result, it receives signal sent to the main process too.
In a POSIX-conformant operating system, a process group denotes a collection of one or more processes. Among other things, a process group is used to control the distribution of a signal; when a signal is directed to a process group, the signal is delivered to each process that is a member of the group.