The whole exercise smacks of utter cynicism to me. Either:
a) he knows he will be banned and kicked out of the International Team, and is therefore pre-empting the decision in order that his wikipedia page will always show he 'retired' rather than was sacked';
b) he knows that the FA can't ever make any decisions, relying instead on 'public opinion', and so having loads of sympathetic media reports appearing ont he morning of his trial will make them less likely to punish him;
c) his lawyers have told him that if he makes a gesture like this, the FA will let it go at that, saying that he has 'been punished enough'.
Whatever happens, don't be deluded into thinking that Terry won't suddenly make himself available again in a few weeks when all of this drifts off the back pages. He'll miss the next two games, but when the next round of games approaches, he will certainly have put himself back in the frame.
This is simply a self-serving exercise in trying to influence the FA board - nothing more.