The US military can sometimes get their weapons cheaper, if there are a lot of foreign buyers to share the cost in various ways.

If Lockheed is successful and able to fund itself well, fund new weapons programs, the military is happier for obvious reasons. Not to mention the crossover between the public and private spheres when it comes to careers.

There is also a follow on effect. If other countries have F-15s, the US needs to have something better. Today we are friends, but that could change (e.g. Iran and the F-14)

It's about jobs, jobs. Programs building weapons that other countries buy keep jobs. And the Saudi's buy a ton of weapons.