In fact, we see that Sanchez has control of the military in the country he resides in (and basically owns the entire country), meaning that his action could be seen as an Act of War.
WMG / Licence to Kill
Heller was being pragmaticHaving worked for the US Government, he knew that there were certain lines you didn't cross. The government would be willing to go down the legal route to extradite and prosecute Sanchez because while his crimes were serious, they weren't a direct threat to National Security. However, the moment he shot down an American airliner with a Stinger missile, he would've gone from being a Drug Lord to an International Terrorist. Anyone directly associated with him would soon find themselves a target for US retaliation.