Trouble is if we don't sell them weapons then they buy them off Russia and China and we have even less influence.
2) If they can easily switch to Russian or Chinese weapons, that implies we're not getting that much influence by selling them weapons.
3) If the US is not willing to exercise its influence (point 1) or is not really getting that much actual influence (point 2), it seems like we're just profiteering off a proxy war.
I could go and rob the local supermarket with that excuse.