Why would we? Sam was a minor character in one episode and never mentioned again. Indeed a character in Trek XI who was originally intended to be Sam was renamed Johnny at the very last minute, in post production because it was felt Sam was too obscure a character. I mean Sam's a character they couldn't even get a new actor or even background extra to play, the guy was literally Shatner with a fake 'stache.Perhaps not. But if Sam was former Starfleet, then why didin't we hear about this before?
I mean, unless you think everything said about Sam in Operation Annihilate is literally everything there ever was to know about him, I'm not sure how now establishing he once served in Starfleet is a problem in anyway.