It's 99% jealousy and because the surgery essentially breaks the status quo for lot of people. You would see in LL documentaries where people would leave passive-aggressive comments like " omg they look like T-rex's now ", " they should have spent that on theraphy ", or " lets see how they feel in 30 years ". Do you really think people type this because they really care about people that might not be comfortable with their height and prevent them from getting the surgery? Definitely no, but it gives them the idea of " if they can do it why can't I? " but with a huge time and money investment that come with this surgery a great majority of people won't be able to get it so they want other people to think the surgery is a waste and/or will make you a cripple.
If you also look at posts where people with dwarfism get like 30 cm ( 1 feet ) worth of lengthening done lot of people comment that it was a good investment and that they are proud of them for doing so. Why? Because even if a person with dwarfism gets it that wouldn't break their status quo and won't effect them, but do you know which other community bash on people getting 30cm+ lengthening with dwarfirm? The dwarfism community, since again it gives them the idea of " If they can do it why can't I?" and break their communities status quo.
Even in this community, funny enough, you would see people in the 160cm range hating on 170cm+ people or people getting bilateral surgeries as well.