This was probably the FBI. And if the data center's switch couldn't mirror the customer port (I can't imagine any data center would use a switch that couldn't - but it is within the realm of possibility), the investigating agency would probably provide a 1U switch along with the 2U server that could handle the mirroring and then they could force the data center to connect the customer through that switch instead.

Just an unfortunate side effect. There are lots of legitimate uses for port mirroring in troubleshooting and monitoring a network, like intrusion detection and performance monitoring.