And as has been repeated many times in this thread as well, keys are not the same things as customers or software license. Keys are a functional part of the infrastructure of gaming, they don't represent a new game.
You can't get keys or renew licenses for all games in the Steam store if merely because you own hard copies of them. It depends entirely on the sort of contract the developer has made with Valve.
What are you even talking about here? A Steam key provided by Steam for the devs = a license for the game. Each key activates one license on Steam for whomever activates it. As you can see in the greenlight FAQ link above, pretty much any new game that gets added to Steam has the option of giving those keys out to their previous buyers for *no cost*.
This is not an example of 'Ow I have a hardcopy (for random game), can I activate it on Steam?', This is about a dev, who got his game on Steam, and as a result has the privilege of creating Steam keys for his own purposes, whether that be for selling on other retailers, giving away, giving to previous purchasers etc. Again, at *no cost*. The only time Valve takes a cut is when Frontiers game gets sold directly through Steam's store. And then it's 30%.
This is not some hidden thing or something. It has been like that for years. It's why those $1 Greenlight bundles with 5+ games still hand out Steam keys to their buyers after they're released on Steam, despite the game having been bought for pennies. (some greedy dev exceptions here and there).
Whether Frontier does this or not is up to them. Doing it will get brownie points, A) because people these days do kind of expect Steam keys if the game goes on Steam, partly because it's such a common practice these days. B) Because they've kept their potential buyer base completely in the dark about the fact that they were even going to be releasing on Steam until they actually did.
As this thread proves, a ton of people would have held off till the Steam release otherwise. This means it was a smart business decision for them to not release on Steam originally, since that gives them 100% cut of those initial profits, however it's going to backlash hard if they don't give those people Steam keys after those people bought it thinking it wasn't going to be on Steam, and right or wrong, that view on the state of things was very prevalent.
And since most people know the keys to give to people don't actually cost anything (apart from getting the effort to get the keys to the people), they will feel let down if such a small effort to make for their loyal fans gets waved aside. Because then people will assume that rather than giving you a practically free key, they expect you to rebuy the game just to have it on the most used platform in PC gaming. (and naturally arguments will be taken to the irrational on both sides)