I'm aware of that.
I'm not saying it's not known by the public; I'm saying it's not officially acknowledged by the US government as being part of their own mass-spying programs.
In other words, it's still officially a government secret, even though everyone knows about it.
Prosecuting someone for a crime based on yellow-dot evidence could force the government to testify under oath about the yellow dots and what they mean... which could force them to answer some uncomfortable questions about how the dots got there... maybe officially admit ownership of the program... and I imagine that could open a whole legal can of worms they'd rather avoid.
In other words, it's still officially a government secret, even though everyone knows about it.
Prosecuting someone for a crime based on yellow-dot evidence could force the government to testify under oath about the yellow dots and what they mean... which could force them to answer some uncomfortable questions about how the dots got there... maybe officially admit ownership of the program... and I imagine that could open a whole legal can of worms they'd rather avoid.