Is "because it's fun" an acceptable justification for choosing a certain research path (when communicating to peers/colleagues)?
There is a difference between the personal motivation of a person to work on a subject, and motivating why a subject is worth working on. We should always be able justify the later, both from an ethical and practical point of view ("because it is fun" will not convince anybody to pay you to do it). 

In practice most researchers (certainly the successful ones), can provide a detailed motivation of why their work is relevant to the furtherment of their field and humanity as a whole.

However, this motivation might differ quite significantly from their personal reason(s) for wanting to work on the subject. In particular, they typically will have reasons for working on this specific thing, rather the long list of other things that are also worth doing. (In this category "because it's fun" is probably preferable over  the much more cynical "because we can".)