[wmf]

Here's a theory: If they promise to capture value of forks then anyone can fork Bitcoin (it's pretty easy to do) and force the custody providers to do a bunch of work to support that fork. By preemptively disclaiming all forks, they also reduce the motivation to fork. (Stablecoins have the same effect on Ethereum.)

Here's a list of 45(!) Bitcoin forks and 11 airdrops that have happened so far: https://forkdrop.io/how-many-bitcoin-forks-are-there

[nullc]

It's hard to make that promise too, what happens when the fork requires that you expose 200 bits of your private key to make a transaction?

What happens when the fork snapshotted the chain at some arbitrary point in the past such that it's not possible to match it up with ownership of the ETF?

There have been 'forks' of altcoins made where the 'official' software was expressly backdoored. They even lead to some fringe exchanges being robbed.

There was a brief flurry of forks when exchanges felt compelled to list them-- they were a way for altcoin creators to avoid the huge listing bribes demanded by exchanges (which also has made it so that only premined altcoins are viable to create anymore)... but after exchanges decided to stop listing them (and esp Archer v Coinbase established that exchanges could just keep the fork coins, if that was their policy) most of the fork creation stopped.

(somewhat to my saddness: diligently dumping fork coins made me a lot of money...)