[nawitus]

Why is returning say the three top comments not RESTful?

[lparry]

Because in rest, urls are supposed to specify a resource, but now your response is a resource and some other resources too.

Not saying that being pure to rest ideology is a good thing. I think it’s a terrible fit for most applications more complex than a demo todo app

[icebraining]

There's nothing "impure" about that. From Fielding's dissertation (emphasis mine):

"The key abstraction of information in REST is a resource. Any information that can be named can be a resource: a document or image, a temporal service (e.g. "today's weather in Los Angeles"), a collection of other resources, a non-virtual object (e.g. a person), and so on."

By the way, REST is not an ideology, it's an architectural style. It defines a set of constraints, and specifies the advantages and disadvantages of applying each of them.