Please....what kind of loony toons would come up with a conspiracy to make killer spiders for use on citizens in a poor country?
The US has plenty of biological weapons to deploy if they need something done, but we are not in the business of genocide or ethnic cleansing.
Besides, those spiders could have migrated simply by being a travel token or hitchhiker in a box, suitcase, etc. Once they arrived these spiders would have mated and expanded the population. There are certainly going to cases where a species alters hunting, feeding and habitual traits in order to survive foreign conditions.