As I understand it, this code is mainly intended for stations that serve as an AP, rather than as a client.  Since the MAC layer is the one with easiest access to the real airtime statistics (much easier than, say, a qdisc), there is no better place to implement it.

If RTS/CTS probes are in use, then the AP does in fact have control over receive bandwidth on a per station basis, at least in theory.  But I think this is simply an attempt to balance total airtime between stations by accounting for airtime used in both directions, even if one of those directions isn't controlled so strictly.

- Jonathan Morton