Counterquestion: why should it be forbitten? It does not write anything to the log, changes no data ...
I don't know how it is realized internal, but theoretical an USE [db] is only a compiler hint, that adds the current used database before any object declared only with schema + object name when creating the execution plan. And since it is fully okay to mix different databases inside an transaction by using the [db].[schema].
-syntax, the same should be true for the "hint" created by the USE statement.
God is real, unless declared integer.