They're really free. They aren't the only ones doing them, either. EA's Origin store used to give away a game every month or two. GOG often gives away a game at the start of their various sales. Steam will give away games on rare occasion, too, though they more often give a "free weekend" where you can play a game through the weekend for free. If you want to keep a "free weekend" game, though, you'll need to buy it. (Of course, if the game is short and you play it a lot, you may be able to finish it in a single weekend!)
The TLDR: So far the games have all been free but more often then not the DLC is not provided just the base game unless it was GOTY edition or something. So grab them with absolutely no guilt.
In fact in the early days on every article that PCG posted about free games i would comment on DISQUS "Thanks for the free games Epic! still not buying anything from your store!" and it pissed a lot of people off for some reason. I didn't care because it was both funny and pathetic at the same time , them not understanding the reasons why i said it it for nearly a year.
Some of the comments were strange and funny though. Some thought that by giving away the games for free we had an obligation to buy from the Epic store. Frankly that was stupid. One person even begged me to stop posting that message (i was usually one of the first to comment) and even tried putting a counter "Thank you for the free game epic! i'll buy from your store!" comments to show how much of sucker that person was.
One person made a weak argument that it was buying my attention and stay relevant (habit forming). In a way it was, but I'm not a steam die hard and I'll gladly take anything that's free. So maybe less then a minute of attention I got a 15.99-30.00 game. I think i came out on top on that one and since my 1 minute attention wasn't converting into anything monetary I think the argument is mute. Even more so that I've taken more then I've bought from the store.
i only stopped when the Epic store finally offered me something i actually wanted to buy and brought in some killer deals. A tenner off anything above £15 was an awesome deal that i couldn't pass up. Even bought Asassins creed odyssey gold edition for £12 compared to buying it full price at around £20 at the time.
But did the free games change my opinion on Epic store? Honestly it probably did. I was content to live and let live, the exclusives side of things did hack me off but when i could see humble were occasionally selling it on their store and the game was coming to steam eventually i didn't care as much. Hell, i don't buy games when they first release anyway. So my resaons for hating them were pointless and they were offering something to Devs who deserved more money. The price gouging didn't happen (which something activision and EA shamelessly do).
But giving away free games to actively try and buy something from the epic store in the state they were in at the time? Sod that.