Because it is historically accurate. The Round Church is a symbol of the political, religious, social and scholar changes brought by Boris (father) and Simeon (son). (think of this as the equivalent of the "free specialist" bonus of the Statue of Liberty).
Boris realized that a non-Christian Bulgaria cannot survive in Christian Europe, so he set forth to convert the country to Christianity. This had the double goal of securing peace with Byzantium and uniting the divided Bulgas and Slavs. At the same time, the Byzantines naturally jumped at the opportunity to extend as much political and cultural influence in Bulgaria as they can. However, Boris wouldn't have that and instead of accepting Greek as the official language, he accepted Slavic and started a huge campaign of translating religious and classical Greek texts into Slavic (philosophies of Plato and Socrates and so on). Hence the +10% science bonus.
Simeon went further than that, declaring himself Tsar (equal to the Byzantine Emperor) and making the Bulgarian archbishop Patriarch (equal to the Constantinople one). This lead to the bloodiest period in Byzantine-Bulgarian wars. Neither side has much of remorse in killing the other, hence the "no war with brother in faith" and "no diplo benefit".
Simeon's son enjoyed a long period of peace, however, as soon as he dies, Bulgaria was invaded by the other Orthodox nation: Kiev. Hence, you cannot have the Round Church and hope for peace and good relations with other Orthodox nations.
http://en.wikipedia.org/wiki/Simeon_I_of_Bulgaria