An attack on a ship inside Bulgaria's exclusive economic zone in the Black Sea is not the same as an attack in Bulgarian territorial waters. The zone starts right where the territorial sea ends, and there Bulgaria does not have full sovereignty — only the rights and powers that international maritime law gives it.
The UN Convention on the Law of the Sea sets the rules for the zone. Under it, an exclusive economic zone can reach up to 200 nautical miles from the lines used to mark the edge of the territorial sea. But the Black Sea is narrow and has several other coastal states around it, so that rule can't be applied word for word. Bulgaria's zone is marked out by agreements with its neighbours in the region.
Bulgaria alone has the right to explore, use, protect and manage the natural resources there — the living things in the sea and whatever lies on or under the seabed. Bulgaria also controls who builds or uses artificial islands, structures and installations in the zone, and is in charge of protecting the marine environment.
Other countries keep certain rights in the zone too, including the right to sail through it freely. A foreign merchant ship sailing there is not in Bulgarian territorial waters, so its presence alone does not break Bulgaria's sovereignty. In the territorial sea, though, Bulgaria holds full sovereignty.
In some cases Bulgaria also has duties and powers in the zone — making sure shipping is safe, protecting the sea, stopping violations, and searching for and rescuing people in danger.
Because the zone mixes free navigation with coastal-state rights, it becomes especially sensitive in wars and other incidents. In a war on the Black Sea, the zone matters for more than trade — it's where shipping routes, energy and resource interests, environmental risks and the safety of civilian ships all overlap.
An attack on civilian merchant ships creates serious problems for the safety of shipping worldwide, for respect of international law, and for how countries in the region can react. That's why it matters so much exactly where the attack on the two ships took place.