Why do you think they did this?
I don't know, but I don't think it was an accident. You really need to be irresponsible as fuck to not be cautious when spending 4+ BTC. Couldn't there be a new kind of Proof-of-Burn perhaps?
P.S. coins sent there are not burnt though
They are definitely not burned. Besides Satoshi spending them (which, between us, I don't find it a wise thing to do), the public key of that address is known. It's entirely possible someone spends that output, once the underlying cryptography is weak enough.