The change streams test suites all utilize the set_read_and_write_concerns.js override as a means to override read/write concern to "majority" on all commands run in their tests. This may have been added initially since change streams originally required "majority" read concern. They no longer do, and so omitting read concern on a change stream query is acceptable. The fact that all commands, not just change stream commands, get sent with read concern majority poses issues for running change stream suites with enableMajorityReadConcern:false, since "majority" reads are not supported for normal, non change-stream commands.