So? Still O(1).

yeah it's a mess

it also looks cooler

You are right, in general. But theoretically, if we are in environment which is restricted by RAM, additional memory allocations sometimes are not desired. Secondly, perhaps authors supposed that caller code is responsible for immutability, by cloning passed arrays, for example. Lastly, cleverness also counts, and sometimes we're sacrificing best practicing for higher cleverness (less code in current case). 😉