While it is just part of the rifle, based on a design posted on cncguns.com, U.S. law considers the lower receiver to be a weapon. MakerBot only deleted the pointer files, but left the description and comments of the submission intact.
It is interesting to note that MakerBot said in February of this year that it would not allow printer files that could contribute to the creation of weapons. In a statement, a MakerBot spokesperson told Cnet:
"MakerBot's focus is to empower the creative process and make things for good. MakerBot Desktop 3D Printers make innovative products, new tools, models, fashion items, works of art, and 3D things of all types. MakerBot's Thingiverse website is designed to be the best place to get and share downloadable 3D "Things." Thingiverse's Terms of Service state that users agree not to use Thingiverse "to collect, upload, transmit, display, or distribute any User Content (ii) that...promotes illegal activities or contributes to the creation of weapons, illegal materials or is otherwise objectionable." If an item has been removed, it is because it violates the Thingiverse Terms of Service."
As it is the case with every other website, MakerBot said that it reserves the right to remove user content, if it violates its terms of service. MakerBot's attorney stated that the recent violent events convinced the company to take immediate action.
A recent demonstration showed that the 3D-printed part may not be a replacement of the original part, but it is good enough to hold up for a few shots.