Sectors are 512, 4096, or another power of 2 because of a combination of 8 bit characters (the lowest addressable unit of storage) and powers of 2 being more natural to fully address when working on a binary based processing/addressing model.
It isn't completely disingenuous to talk about intra-systems communication in terms of 'million bits' (engineering mega, not binary 'mega', bits) but it is obtuse and full of marketing spin.
I've never seen any source indicate that SI defines units for data measurement. I've only seen such standards coming from organizations like IEEE, IEC and JEDEC, and those standards have not been all that consistent over the past few decades.
The question in this case is the meaning of the prefix, not the unit to which it applies. SI defines these prefixes as powers of 10, regardless of what it applies to.
It isn't completely disingenuous to talk about intra-systems communication in terms of 'million bits' (engineering mega, not binary 'mega', bits) but it is obtuse and full of marketing spin.