Why Washington counted battlecruisers with battleships

Why did the Washington Naval Treaty count battleships and battlecruisers together?

Because only Britain and Japan had battlecruisers, while the treaty's other principal negotiators had little reason to preserve a separate category.

The Washington Naval Treaty treated battleships and battlecruisers as one capital-ship category rather than preserving their doctrinal distinction. The conference politics explain why: Britain and Japan were the only signatories that actually possessed battlecruisers. The United States approached the negotiations without an existing battlecruiser force that needed separate protection. A combined tonnage category therefore simplified the ratios for most powers while forcing the British and Japanese fast capital ships into the same arithmetic as battleships.