Why is line go up price discovery acceptable, but line go down price discovery not? If the shares are trading, you should be able to short, it’s arbitrary to disallow it. It is quite literally a part of the market settling on a price.
(under the assumption your broker is managing their risk if your losses from a short position potentially exceeds capital available for liquidation if the trade moves against you)
Line go down discovery is acceptable (that is what selling a share is). The reason you might not want options trading very early after an IPO is because the market is frothy enough without the additional layer of complexity.
Certainly, its reasonable for a delay in options being available while market makers prepare to make the market for those options. But shorting? Day 1, the shares are trading and available to borrow to sell to short.
Are they actually? How many intermediate steps are involved in finding shares to borrow for a short? I imagine they have to be transferred to some central depository with the feature, for a start, and that takes 2-4 days
Your broker will locate and borrow the shares from an available pool (such as other clients' portfolios), sell them on your behalf, and hold the cash. They don't have to go to the clearinghouse.
Because lines tend to trend up over time. You're betting on lines going down, and paying rent while doing so as shorting requires you to rent/borrow shares from somebody else. It's an extremely high risk activity that can easily result in an investor losing a very large amount of money.
Elon Musk is politicized so you're going to have people wanting to short against him, for reasons other than it being seen as a rational and sound investment strategy. This is one reason brokers tend to restrict this activity to certain types of investors who are more able to appreciate the risks, to say nothing of baseline necessities like needing a margin account to cover potential losses. Shorting is just very different than buying a stock.