1200 is it ever ok

I see a lot of people trying to eat to or under 1200 calories. Isn't that under most people's BMR? Wouldn’t that be slowing down their metabolism, causing plateaus ect? If it is under MFP's calculated BMR then why does it allow people to set that as a goal?

Some friends don’t share their diaries or their goal is that low. We congratulate each other for it when it may ultimately be sabotaging themselves. They feel bad when they "eat too much" but are likely still under their TDEE. I think our ultimate goals are to lose weight healthily and keep it off...