It's reserved because the donation was earmarked for a specific purpose (eg. a business program or whatever), not because they reserved 30% on tax owing.

>Is the growth 'reserved for a particular purpose'?

It's probably safe to assume donors are competent enough that such glaring loopholes don't exist. After all, the concept of endowments being used as long term savings, rather than spent immediately, isn't exactly a new concept. Failing to take this into account would mean any earmarks are void after a few decades.