Its a little bit more complicated than that. US exercises its veto mainly when there is a competition against an American product. Even Gripen exports have been affected when US denied them in cases where F-16 or F-18 were competing for sale.
F/A-50 sneaks through because its a different category of aircraft. It's genuinely Light, has limited value as a strategic asset and is low in terms of armaments capacity compared with Gripen. Price wise also it's very attractive for Tier 2/3 operators like Philippines who could never afford an F-16 for eg. That's the market that TAI has successfully tapped into and US for the most part doesn't interfere as they are not directly competitive with any American product.
Tejas as an export will do better if it is market in this same category. The current Mk.1 configuration (once its mature) is perfect for most countries that do not have any direct adversarial relationship with their neighbours but require a basic air defence and CAS capability for sovereign protection. In our immediate neighbourhood, a country like Sri Lanka, Myanmar, South Africa, Indonesia, Oman all would be good targets for sale.
But again, both HAL and the IAF need to stop their infighting, get their house in order and start churning out operational aircraft. As long as the plane is not seen in service with the IAF in substantial numbers, and without the IAF constantly moaning about its deficiencies in public, the export market will never open up.