Because it was written by a dummy, me :-)
Should have called it MockTXMgr...
For http session replication, the cache is using BatchModeTransactionManager in order to segregate the tx used for session replication from any user transaction. (This necessary because if FIELD granularity is used we start a tx on request entry).
I'm assuming we didn't want our TM named "Dummy" (or even "Mock").
The DummyBTM and DummyTM are named such so that they aren't used in production. They are in no way production-grade transaction managers and we would always recommend using the TM that comes with your app server (or a standalone one)
Regarding the BatchModeTM, it is used in PojoCache to simply batch up operations - a subset of transactional symantics. The class name suffix 'TransactionManager' is a bit misleading in this case though. I'll update the javadocs to reflect this.