It definitely is - one store, Epic, offers deals publishers can't refuse to make a game exclusive to their storefront which limits a consumer's ability to use a platform of their choice to purchase a game. The other, steam, puts no limits on where you can distribute your game (unless you use assets from Valve's own games, but I digress) and has done no exclusivity deals that would force a game to only use their store. Publishers are free to use steam if they want to have 30% taken, but otherwise they can completely forego steam in their distribution.