Whilst the concept would be one most of us would buy into, it splits the MP pools further as not everyone will buy them and personally I think if you update a game over time and paid our money upfront like we have, I'd expect the maps to be given for free when they do get released.
I noticed it straight away so went to look at what it meant last night before posting it here. I think we knew most subsequent maps would come off the larger Warzone map, but that makes total sense from a developer viewpoint as its the same assets they've worked on and already in the game so doesn't make the game any larger content wise. For a 2-3 month Season, i'm still holding out that we'll get at least 1-2 more maps they haven't told us about mid-season like we did in Season 1. If that is the case, then I'd be okay with that. If they don't release anymore maps mid-Season then i'd agree that its disappointing.