The reason why this drains entropy is because when you start a new process (for example "cat" in this case), the kernel passes 16 bytes of randomness at exec ti

The reason why this drains entropy is because when you start a new process (for example "cat" in this case), the kernel passes 16 bytes of randomness at exec time for the benefit of userspace. This allows for fast stack randomization, without the overhead of explicitly opening and reading from /dev/urandom. Due to the internal kernel interface which gets used, this draws from the /dev/random pool, thus decreasing entropy_avail, although it is not a blocking call. There has been some discussions that the userspace randomization should use a CRNG, or perhaps draw from the urandom pool instead.