It's defined in the umbrella Java EE spec, see also "EE.5.24 Support for Dependency Injection":

Therefore, to make injection support more uniform across all Java EE component types, Java EE containers are required to support field, method, and constructor injection using the javax.inject.Inject annotation into all component classes listed in Table EE.5-1 as having the “Standard” level of injection support, as well as the use of interceptors for these classes.

Years ago, I sent a question to the cdi-dev ML: http://lists.jboss.org/pipermail/cdi-dev/2013-October/004345.html