Well they also aren't bad things: they count for two likes and are used to indicate sadness, not disapproval. According to articles on the ranking decisions, Facebook internally calls them "Sorry". You don't want to take a post where someone says something sad and people react with sadness and say "this is bad content".
I think you're confused. Facebook has both a "Sad" reaction and an "Angry" reaction. They can both be used to agree with and disagree with the recipient, depending on context.
Ah, and you believe the original post here was talking about Angry, not Sad? (I have a hard time reading "frowny" in general non-Facebook contexts as "angry" instead of "sad", but I also have a lot of weight using the term over the years as part of "frowny pants" and so might be biased.) FWIW, all "reactions"--including Angry--were worth two Likes when they were first introduced (maybe maybe there was one that was worth 1.5? though I think it was all 2.0), but Facebook recently downmodded Angry (only) to be worth zero (but not act as a counter-signal). And, as you seem to understand--though I feel like you fail to appreciate the gravity--neither are inherently negative reactions: if you post something about vaccines (say, to choose something that people get super upset about) and you get a thousand Angry reacts, there is literally no way to know if those people are angry at YOU for posting the article or angry at the CONTENT you posted (commiserating with you), and so this mechanism simply doesn't serve as a "disapproving glare" as it isn't merely context-dependent but subjective in meaning.
The reactions I see are a sad crying sad face with a single tear and an angry red frown. Neither of those would be described as a frown. You are right that neither is something that could be used to tell someone they are being inappropriate.