My guess:
T&C state (my bolding) "An award ticket may be reissued, but only for travel by the same person and provided no part of the award ticket has been utilised." Note, if you add a stopover ticket would require re-issue, not just revalidation like date/time changes.
It seems what they might be doing here is getting around this rule by splitting the trip into two one-way awards rather than a round trip award, therefore if you add a stopover to a one way award then a US$100 fee applies.
This all just speculation though, no experience of the process.