You misunderstand that “exposing yourself to the language” is not the same thing as getting poorly timed memrise spoilers, even though it can seem the same for an individual instance. Memrise is supposed to be, in a way, detecting how well you’re learning, including detecting how much exposure you’re getting to what you’re learning in real life outside of memrise.
If you’re learning a language and actually speaking it, or reading books in it, etc., there are words that will appear there that you have a natural exposure to. In those cases, memrise will detect that you know them and, overall, schedule things appropriately. Yes, there’s a slight chance that some word you hardly ever encounter in real life will appear just before you do a memrise review, just once, and then you won’t see it again for months, and in that case memrise’s algorithm will be fooled and your scheduling won’t be optimal. But that won’t happen much.
The problem with memrise spoilers is that it’s not a coincidence. You’re getting the spoiler because that word is in the memrise course you’re taking. Probabilistically, this will mess up your schedule pretty often, because it’s not a coincidence. It’s not that the word is rare and just happened to pop up, it’s that the word is in the course and popped up because it’s in the course, but it popped up in a way that prevents memrise from being able to learn how well you know it. Memrise is supposes to cause words that are in the course to show up for you, but it’s supposed to pair up that reminder with detection - that is, memrise needs to know, every time it shows you the word, whether you know it now (just before it actually shows you), and how long since you last got tested on it. Preview does an end run around that process.
Perhaps this would be clearer if I used my tropical fish identification course as an example. In it there are fish from various regions of the world. Because of where I live, I go snorkeling or diving in the Caribbean or the Gulf of Mexico a few times a year, but have never even once made it to the central Indo-Pacific, where many species are based. So, for Atlantic fish, there are a number that I am quite likely to see in person a few times a year, and thus I learn those much better. Some of them I know very very well, due to seeing them in person numerous times. Memrise detects this appropriately, and since I nearly always get those fish right, they stay on a 180-day review schedule.
If I preview a level, I’ll see the Atlantic fish on that level, and also Hawaiian fish (which I may have seen in person but not for over a year), eastern Pacific fish (ditto), and Indo-Pacific and Indian Ocean fish. A few of them are present at the aquarium near me and I have gotten to know them, but various others are not, and I know them only from making this course. If I preview a level, it doesn’t matter which fish I see or know in real life - it’ll show me all of them. If any of those Indo-Pacific fish I barely know come up for review soon, I’ll get them right, they’ll be scheduled for further in the future, and I’ll forget them, because I really needed a sooner review, but memrise doesn’t know that. I fooled it by doing a preview.
What’s significant here is, the reason I got spoilers is because those fish are in the course - that’s why preview showed them to me. Even a very rare fish I might hardly ever see in person, and might take many years to learn well, would show up. It’s not a coincidence, it’s not real life, it’s not something I’m naturally going to learn because it shows up in my life sometimes.
If that happens once, no big deal. It’ll happen on rare occasions with rare real life fish/words, too. But if you use preview, chances are it’ll keep happening, much more frequently than it naturally would. Fish I never see in real life, show up just when I preview. At that point, I might as well be using the “cramming” method of learning, and try to ensure I preview things often to get them to stick in my memory; memrise’s schedule based algorithm is effectively being disabled.