The random number generators used by these sites are hardware systems that use micro fluctuations in ambient temperature (for example) as a source of entropy and they are very careful to use enough bits of entropy for every card shuffled.
It's amusing to realize that they could just read from dev/urandom with zero risk. They're probably not running Linux, but still.
So, for anyone who's wondering if you need this, or if this adds any extra security: probably not. There's no reason not to use the extremely well-tested and well-understood /dev/urandom.