Except there are plenty of life altering surgeries cis teens routinely undergo, even cosmetic surgeries, and nobody seems particularly worried about them. Moreover, these laws usually carve out exemptions for cosmetic surgery on intersex infants’ genitals… I guess permanently altering a child’s genitals is fine as long as they can’t ask for it.
As for medication with “potentially irreversible effects,” that’s true of puberty too. Forcing a trans teen to undergo natal puberty is forcing them to acquire secondary sex characteristics that either take expensive treatments or surgery to reverse, or are irreversible. Puberty in general makes irreversible changes well before someone is 18. Being uncomfortable that trans teens exist and wanting to kick that can down the road until they’re legal adults doesn’t mean that’s actually what is developmentally or medically appropriate.
As for the FDA thing, it sounds like you’re referring to off label usage. Every drug has them. It doesn’t mean nobody knows what it does, puberty blocking drugs still block puberty whether you use them in a trans teen or a kid with precocious puberty.