From that letter: "each profession has its unforgivable sin, and in sports it is doping." That simply is not true in sport. As the BOC found out with Dwain Chambers, doping gets you a time-out, not a ban. See Contador.
He has no chance of countering the allegations because he cannot prove he is innocent and the USADA has convicted him before the trial based on hearsay evidence. There is no point in him contesting it.