"Monks" includes everybody? I have often heard that in many places in the suttas where the Buddha says Bhikkave, (monks!), he really means to include the four parisā (assemblies), which includes lay male and lay female devotees. Therefore, it is said, that teaching applies to all. 

Where did this idea get legitimacy? Is there any mention in the canon itself? 

I can readily see why one would wish it were so (why would one ignore all the wisdom because it's not spoken directly to oneself) but that doesn't mean it is so. 

I'm also interested if this explanation is offered in Mahāyāna where the meaning of saṅgha itself is broader, as I understand. Also within Insight/Theravāda circles, is there a difference between the West and traditionally Buddhist countries like Myanmar, Sri Lanka, Thailand in the reading of this word. Any historical insight into whether the West was influenced by the early arrival of Mahāyāna to its shores, for example. 

Edit:  I am aware that it makes sense to view the teachings as applying to everybody. Please note that the question is more specific - is there canonical support (in any canon) for this interpretation? Or some other historical information other than using our own logic.
It is an effect by wrong way commentary reading of the western professors and thai professors, because they have not enough skill to read pāli tipitaka and commentary.

Commentary just want to said:


  Buddha called "bhikkhave" because the listeners in front of buddha are bhikkhu. But it doesn't means the other buddhist people can not practice follow that sutta.


But in the appearing commentary doesn't said  "because the listeners in front of buddha are bhikkhu", like above, because it is simple in mukkhapātha tradition to call just the specify target, most cases are the nearest. And the most buddha's nearest people were bhikkhu.

Today, you can see in the classroom, when the teacher ask someone to teach 
, such as "Peter! don't break the school's rule", it doesn't means "just that one can't break", but it means "everyone in school can't break". It is simple, so it doesn't need to explain as "the others can't break, too."

There are some excepted cases that should use the difference meaning of "bhikkhave", such as in 1st vinaya rule. For those, there had the explanation in commentary, case by case. It does't means "commentary commented by there own opinion", but it means "that sutta's context or the other related sutta make the commentary commented like that". Because the commentary teacher must be tipitaka memorizer in ancient theravāda tradition, such as ancient mahāvihāra monastery. 

I don't want to blame the others, but I must specify an actual main problem cause of new age buddhist students, that make them feel so hard to study buddha's teaching.