Found the latest discussion in the
FAQ/Errata thread starting at post #111
I don't believe the errata or FAQ draft has been updated to clarify which way it should go, but we left it with INF getting the bonus of the unit they are riding. INF need to explicitly dismount to get their own terrain bonus, otherwise, they share the fate of the unit they are riding. That's kind of the point.
Since INF riding *in* a unit came up, it's worth also addressing that. The only place that occurs is with [hover]trucks. Regardless of how the rule for riding ON a unit gets resolved, it's pretty clear that INF riding IN a unit have no way to get a terrain bonus that's different from the unit they are inside.