Previous company actions only really belong in the previous company environment.
That is, champions and notions arising from fan concepts years ago may no longer be prudent today. Be it legal reasons or otherwise, they've decided now that they won't source the community for ideas. Legal reasons is a good one, frankly copyright claims of any kind just puts a kink into their operations. Even a disingenuous claim can be enough to eat up money and work hours, far beyond whatever profit they may get from the idea. It's not solely about what players offer up: someone may generate an idea that is disturbingly close to another company's intellectual property. If one of those slips in and the company takes action, it'll just get ugly.
Keep in mind there's a distinction to be made between talking about 'fan creations we like' versus 'fan creations we'll incorporate to our IP'.
Creations we like: Hey, the community wants a female tank (Leona), we'll take some general ideas about heavy armored women and recreate it in our style.
Creations we'll incorporate: Hey, Veigar is actually a pile of furby-like creatures, not a yordle (as done by Artist X), we'll make that canon. (Artist X demands recompense for having their idea implemented, Riot argues its their IP and all fan work belongs to them by default, etc).
American copyright is a hellborn war zone of 'he/she said, they said', with major companies trying to squeeze out people making fan work of their property. The Disney Corporation is notorious for this, and perhaps one of the most visible examples of big money squeezing copyright laws around. The second you start involving yourself with other peoples' ideas you don't have under a legal ownership, you can expose your company and property to all sorts of legal problems.
Hence, the question is always: Is this one idea worth all the legal risk?
Some sub-variations: Is the person who made this idea cool with it? Will they cause problems if we sanction their work? Will their work reflect badly on our company if they use it as validation? etc.
We haven't even touched on the subject of accidental empowerment and the drama that can create inside a community.
So, in the end, there's a lot of land mines to consider as a company member when you touch community generated ideas. No one wants law suits, or to be the one that causes a rift by making Person Y really popular.