It's likely a trademark issue. A trademark is suppose to be an adjective (a Google search. A Lego brick.)

This is because a trademark identifies a specific source or producer of an item. If a trademark is used as a noun (Kleenexes, Trampolines) it is a step toward generic usage and loss of the trademark.

Hence, the Lego group would want you to talk about Lego bricks and Lego pieces, but everyone else is fine talking about Legos. (Except my autocorrect, which wants it to be Lego's)