Are keynote talks being purchased?
Some conferences exchange corporate sponsorship for various perks. Those perks may include a keynote speaking slot. Thus, 


This kind of thing happens in academia.
Perhaps more so at serious conferences, because those conferences are more attractive to sponsors.


Perks may be advertised, e.g., on web pages soliciting sponsorship. Hence, 


This may be disclosed to conference participants.
But, possibly not in a reasonable manner, since participants would need to discover sponsorship perks.


Some perks may be privately negotiated, without disclosure to conference participants.