Should sources of sample sentences be shown in (computational) linguistics research?
I believe your field is doing itself a disservice by not citing the sources of language utterances. Particularly if direct quotes from published sources are used, the information about the source is important. Did the sentence come from the Los Angeles Times or the neighborhood community newspaper? From a novel written by a Novel Prize winner or a first-time novelist? From a book written in 1879 or 1979?

It feels like an archaeologist writing about a pot without including where it was found, what was found around it, etc. 

The field is just not practicing good science. 

If you feel that your paper will not be accepted with more complete referencing of the language utterances, try to mitigate the issue by using examples from the fewest references possible or creating your own with the same structure or pattern.

I think you've found a nice hole in the field--can research be done to show that omitting or not studying the sources of the language utterances changes results?