If Russia had 30% of people who can understand only Estonian, I think it would be unjust not to give them citizenship automatically. Am I crazy? Did any other country, except Estonia and Latvia, force 1/3 of its population to learn second language and pass exam to become citizen of country where they already lived for years?
30% is the number of "ethnic" Russians in Estonia, but at the time being only 7.5% of Estonian population is "stateless" (meaning of Russian descent that did not get citizenship).
Most of the "ethnic Russians" in Estonia got the Estonian citizenship, and others acquired it later on, often automatically (people completing schools in Estonia do not need to give an exam to demonstrate knowledge of Estonian language and constitution).
So, quoting 30% is misleading to the facts.
Estonia didn't grant automatic citizenship to a specific subset of "ethnic Russians" because according to Russian law they could be granted, upon mere request, of Russian citizenship.
Estonia wanted to avoid or at least minimise the number of people with dual citizenship.
Part of that 7.5% did not want to get Estonian citizenship to don't lose the Russian one.
For personal experience I know that Russian Federation does not allow double citizenship but apply a "don't ask don't tell" policy: as long as you don't tell them, they don't care if you got a second passport.

Even knowing it would entice a risk for those who wanted to freely travel to Russia without having to bother with their visa process.
For Estonia it's a huge loophole for double citizenship.
Another part of that 7.5% didn't take the exam because it's difficoult to learn a new language, especially when age advance.