Isn't it very common for a currency to be pegged to another, though? Dozens of currencies are pegged to the EUR or USD, and the two CFA francs are not very different, except that the French treasury is legally bound to guarantee free convertibility between CFA franc and euro, unlike say the US federal reserve with Cuban peso.
Yes, it is common. There are even countries that officially adopting a currency they do not control, e.g. Croatia uing the euro. But this has the very real drawbacks that the previous comments mentioned.
> It's their own national bank which buys and sells tons of Euros to keep the exchange rate stable.
The two CFA franc zones have zone central banks (which aren't exclusively controlled by countries in the respective zone, since France has a seat on each), it's not individual national central banks issuing currency or managing the peg.