Thirty five years ago, different countries had their own telecommunications infrastructure, so the division between foreign and domestic collection was clear. Today there's a global communications infrastructure, so there's a greater risk of collecting on Americans when the NSA collects overseas.
One interesting aspect of the revelation is what it suggests about the interplay between the other vast collection program involving Google and Yahoo: PRISM. Here's what Gellman writes on that point:
The infiltration is especially striking because the NSA, under a separate program known as PRISM, has front-door access to Google and Yahoo user accounts through a court-approved process.
The MUSCULAR project appears to be an unusually aggressive use of NSA tradecraft against flagship American companies. The agency is built for high-tech spying, with a wide range of digital tools, but it has not been known to use them routinely against U.S. companies.
Google and Yahoo told the Post they were unaware of MUSCULAR's infiltration.