It's about competitiveness. I'd say calling that was chickensh!t, but there are some people for whom not losing is the prime directive.
And I'd say the round was over so spiking the disc could not be a practice throw or whatever complaint was given.
I wouldn't make the call but while Cale may have been done playing, others on the card may not have been and the celebration could have been distracting which could be considered a courtesy violation.