This is a many-to-many merge join, where the execution engine must keep track of duplicates from the outer input in a worktable, and rewind as necessary. Duplicates? We are scanning a unique index!
It turns out the optimizer does not know that a filtered unique index produces unique values. This is a one-to-one join, but the optimizer costs it as if it were many-to-many. The higher estimated cost of a many-to-many merge join explains why a hash join plan is chosen.
Read the whole thing.