Since you never wait on I/O and don't take locks, optimizing execution makes a lot of sense actually. All that's left is execution and network.
I'm not sure where you are getting this information. According to the MemSQL documentation, they have one and only one isolation level, READ COMMITTED. But READ COMMITTED takes write locks and blocks reads, but doesn't take any read locks. Blocking behaviour still (and should!) still occur.
It sounds like you are looking for dirty read behaviour, but from the literature MemSQL doesn't support READ UNCOMMITTED.
Update: Read a later FAQ on the MemSQL website. They are using MVCC, so write locks are irrelevant. My apologies to the MemSQL team.
Isolation levels are used to specify semantics only. DBMS is free to implement them in different ways.
Take a look at this VLDB 2012 paper about optimistic concurrent control in main memory databases, like MemSQL, which also describes implementation of isolation levels: