Clarify decorator beans relationship to EJB spec

In "8.1. Decorator beans" there are two following sentences:

Decorators of a session bean must comply with the bean provider programming restrictions
defined by the EJB specification. Decorators of a stateful session bean must comply with the rules
for instance passivation and conversational state defined by the EJB specification.

I guess the first sentence corresponds "somehow" to "4.9 The Responsibilities of the Bean Provider" in EJB spec. This EJB chapter (specifically 4.9.2) specifies e.g that session bean class must not be abstract.
This seems to contradict bit with abstract decorator for session bean, which is legal in CDI. This restriction would be IMO unnecessary.
I am also not sure about implications of the second sentence.