1 I'd say they came to him
Most likely he met up with the co-author, a surprise surprise Sunday Times journalist, who had spent 2005 with the Tour writing his eulogy of Armstrong while not spotting anything untoward, and about Hamilton, Ulrich etc. There is an interesting comment in an Amazon review of one of the two virtually identical books he published on Armstrong:
"We see how Armstrong has to contend with more than just winning the Tour de France; he has to contend with the multitude enemies, seemingly lead by journalist David Walsh, that are just out to get him: those that want to `prove' that he took performance enhancing drugs and in doing so make there own fortunes."
2 if it's not fact he and the publishers would be on a defamation charge quicker than you could say 'litigation' so the publisher's lawyers would've wanted more than enthusiastic testimony before they went in to print.
Maybe or maybe like David Walsh's publishers they did the calculations. But from the reviews so far it seems he's accused everyone in cycling of that era and made himself a victim of them.
Whatever is in there though was clearly not credible enough for the Grand Jury to proceed with a prosecution of anyone.
3 the publishers know what sells copy. He may have talked for hours on training regimes, alpine routes and daring descents but all they'd publish would be transfusions, doping and stage wins.
The co-author knows what sells to publishers. It appears he spent 18 months interviewing Hamilton before writing it and had planned to launch it on Armstrong's birthday (nice!) but then the story broke and they brought it forward.
With the vested interests of ASO, the UCI, sponsors and TV companies do you really think these guys felt they ever had a choice? Publishers or whoever, I'm sure the corporations were making more out if doping than all but the biggest of (texan) riders.
Are you saying Armstrong was a victim too? After all he could have had no expectation of seven wins (which is what made all the money for him) when he was setting out for his first and second win.