If your 'stateful actions' are EJBs, then that's why.
Yes, we discussed this in the EJB EG and at the time we decided that we wouldn't try to eliminate the requirement for interfaces. I now regret that, esp. since in the context of Seam you find yourself writing interfaces that are never actually used anywhere (the only client is the EL, which is untyped!)
Hopefully in the next rev of EJB, we can fix this.