In what sense are UCI Pro teams 'national' teams? This page on wikipedia lists the UCI Pro teams and has a column listing their Country. It is sourced from this page on the UCI official site. What significance does this hold? Clearly each team doesn't have an obligation to only include riders from that country.

What is the source of the association between teams and countries?
The association is based on where the license holder of the team is located. Garmin Sharp is listed as USA since they are based in the US. Omega-Pharma (OPQS) has their license held by Esperanza bvba, which is based out of Belgium so the team is listed as being from Belgium, etc.