I think this is the main thing. The US sports federations and the USOC have NO formal connection to the US government, unlike many countries. This means that the US government has no control over individual athletes of sports groups in the US unless they break US laws/regulations or something of that nature.
In Russia and many other countries (even relatively clean ones), the government directly controls the Olympic committees of that country and have influence over the administration of individual sports. In most cases, the government has almost no control outside of the Olympics, but in Russia this is not the case. Because of that, a blight on more than one sport makes it seem like the entire athletics administration there is corrupt. That's not necessarily true, but it's the image that's portrayed in such a case.
As I said in the thread about B/K, the reason Russia is getting increased scrutiny is because they have a history of institutionalized doping and one that is currently being investigated and evaluated for a scandal just last fall. If a country, federation, or athlete has a history of doping, they're under increased scrutiny; if they have a recent history of doping, that scrutiny will be even greater.
The US and Australia don't have a history with institutionalized doping known to WADA, just one with individual athletes and sports. So WADA has increase scrutiny on the individual sports and athletes, but not for all athletes from that country. Russia is under investigation for doping in many sports supported by a national lab, making that doping seem institutional, so WADA wants to know more about doping in all Russian sports, increasing scrutiny. Particularly since this doping scandal may affect the entire country's participation in Rio's summer Olympics WADA wants to see how widespread this is in Russia, meaning increased testing of all Russian athletes in all sports.