Very likely because public search APIs are a huge target for bot activity. For reference, about 99.5% of the search queries that goes toward my search engine i

Very likely because public search APIs are a huge target for bot activity. For reference, about 99.5% of the search queries that goes toward my search engine is demonstrably bot traffic (excluding the traffic that goes to the free and public API endpoint I offer). A capable search functionality isn't exactly cheap to run, so requiring a login is the obvious way to be able to rate limit searches, which is otherwise impossible against a botnet.