I have heard this argument a number of times since starting to hear honor cases in the last few years. The simple and direct answer is the old adage "When in Rome, do as the Romans do." (Thankfully I don't have to dredge up a source for this saying, as stuff that is in common use does not have to be cited.) If you had teachers who told you to assemble papers without listing sources, they did you a grave disservice. Perhaps they thought that this was a good stepping stone to writing, that giving you a place to start with someone else's text would eventually lead to writing on your own (sort of like using training wheels on a bike). I have no problem with this technique as long as all the sources are being cited. I doubt that paper you write will have the polish or insights that come from figuring out how to say it yourself.
If you came from a country where the definitions of plagiarism are different, then you have some catching up to do here. Knowing how and when to cite is your responsibility. However, any professor will be happy to help you do it. Almost all of us wrote Ph.D. dissertations that were full of footnotes, references, and bibliographies. All the academic works we are now writing have them. So let's hope we know what we are doing, although I have to admit there have been egregious cases of plagiarism even among faculty members (not at Georgetown, of course!). If you are in doubt, ask. If you need more help, ask the professor if he or she has any objection to your getting help at The Writing Center, and take advantage of it.
There are some professors on this campus who will tell you that plagiarism is a chimaera. There are no original ideas left anyway, and all we are doing is manipulating sets of symbols. (I have probably grossly misrepresented their positions.) Such philosophical positions notwithstanding, plagiarism is a reality if you don't cite the sources. It's that simple.