It's pretty likely that it's just buggy code that happened to work on hardware.
Another example: there are several GBA games that dereference null pointers, but it works because there is no memory protection fault hardware in the GBA. Since the BIOS is mapped to address 0 and yet locked out from reading post-boot (in a failed anti-copying mechanism), you have to emulate the open bus behavior to run these games correctly as a result.
There are also several anti-emulator routines in the GBA library, such as lying about the type of save memory (flash, EEPROM, or SRAM) that a given game actually has (by including strings in the ROM from the Nintendo SDK used to access them.)
Maybe there was supposed to be music playing, this was for timing it, and it was removed, broken, or just forgotten about?
Did they have software emulation for that hardware at the time the game was written?
I understand it was less practical to do this for say, the NES or SNES. But it should have been done for the GBA. It's not just important for future hardware to be able to easily emulate GBA games, it also protects against product refreshes breaking games, something that plagued quite a few classic Game Boy games as newer models came out and fixed bugs in the older models.