I dont think it's unsportsmanlike at all. I've been on both sides of this and in all cases it's just a case of forgetting to vote (esp smaller teams, where not many comments keep it alive). Some sort of system of reminders for VC games getting close to time limit will almost certainly almost eliminate this problem. (IMO)
Team Vote Chess Forfeitures
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I wonder if statistics are kept on Team Vote Chess games in which one team forfeits by failing to move before the TL lapses. Recently 2 teams have forfeited vs The Killer Derivatives in this manner, and coincidentally both games were still interesting and far from over. And this was 2 out of about a dozen games I'm playing on TKD teams, admittedly a minuscule sample here.
At any rate my somewhat long-winded pt is if this is a common problem I believe teams with a certain percentage of forfeit losses (to be determined by chess.com) should be banned (at least temporarily and maybe permanently if they keep on forfeiting after so many temporary bans) from fielding more teams this is unsportsmanlike behavior analogous to players who let their clock run out in losing positions in live chess, isn't it?