Contador claimed tainted meat as well after he tested positive for clenbuterol. But the positive test was very, very, very likely because he used it to help keep or get the last bits of weight off in the weeks leading up to the Tour. He also likely assumed he would take very little out of competition he wouldn't get caught, and didn't realize how sophisticated the testing was becoming.
Froome tested positive for salbutamol, which is an asthma drug he has a TUE for, but the amount he had in his body was not realistically possible just by using his inhaler an extra time (or two) like he claimed. It's very likely he was on a very refined micro-dosing program, including ingesting this, and got caught, in a similar way Contador (and others) have. His and Sky's attorney's buried anti-doping authorities in a mountain of legal paperwork, which intimidated the UCI, who was already very worried about the reputation of the sport taking yet another hit, and losing millions of dollars, and allowed him to skate.
Landis has stated that he didn't actually use testosterone before his epic Tour stage, but it was really blood boosters. He did admit to using testosterone before, though not much, and it was speculated that the + test came from re-infused blood, which had been drawn from a time when he was on testosterone. Floyd had no thoughts on this. FWIT, I generally take Floyd at his word.
By far, BY FAR, doping tests yield false negatives, much, much, much more than false positives, which is how it should be. So if you think an athlete is clean because they were never caught, you may, or may not be deluding yourself, but it's better to give them the benefit of the doubt and have a system that does the same. If you think an athlete that tests positive is actually clean due to flawed testing, it's highly, highly unlikely. Possible? Of course, anything's possible. But likely? Not even close.