This excerpt is from David Hucaby CCNP Switch - page 356, "when the local SPAN is enabled, STP is disabled on the destination port. This allows STP BPDUs to be captured and monitored but also allows the possibility for a bridging loop to form. NEVER CONNECT A SPAN SESSION'S DESTINATION PORT BACK INTO AN ACTIVE NETWORK". If STP is disabled why can't we connect port back and what disruption will it do to the network?
I believe the answer is exactly what you've mentioned
STP is disabled.. Remember that without STP, bridging loop form. If you're adding a port which cannot be configured to work with STP then this port will remain open and BPDUs can come in from this port, when they are not supposed to.