Yahoo is simply the first company that we publicly know of that scans incoming emails. If the government can force Yahoo, they will force all of them. Yahoo has more disgruntled former employees to reveal secrets than Google, Microsoft, etc...
To be fair, according to this article, Google, Microsoft, Apple, and others have explicitly denied allowing government officials direct access to their servers. However, due to the secrecy requirements of the law, we cannot know if they did allow the government some other form of access that was functionally equivalent to a rootkit on the server in terms of what the government could access. Pure speculation on my par…
And the government can compel them to lie about that, too. So their statements are literally useless and cannot be trusted. Period.