Lots of people with agree with you. But your main point is semantic, and what you write is in conflict with the Wikitree Help Page on sources:
“A source is the identification of where you obtained the information.” (I’ll set aside the point that almost no one I know would think that a source is a type of identification, as this sentence states.)
To take your example, if someone copied directly from an Ancestry tree into Wikitree, with no further verification of the information, then according to the Wikitree definition the Ancestry tree is the source. This may be just sloppiness, but as Mike said, it may be from private records and it was the person putting up the Ancestry tree who is sloppy if they don’t say so.
If you then copy a birth date, work really hard to document it and turn up nothing, and then cite the unsourced tree, you need to give that tree as the source. IMO writing “Ancestry tree” at that point is like saying it’s at the library, while giving also the tree name, user, url, access date, etc. is no different then citing a crummy, unsourced compiled genealogy sitting on a shelf in a real library.