As all Red Bull drivers he's been nicely funded by Red Bull through his career and now he needs to pay that back. Vettel was also on a low retainer when he first started in F1 and joined the Red Bull teams.
I think Vettel was on relatively low basic pay throughout his time at Red Bull wasn't he? For a world champion anyway. I thought one of the reasons Ricciardo was a cheap option in 2014 was that Red Bull might have to give Vettel a significant pay rise to match a big name signing? As you say, Red Bull put their money and their faith in the kids, but in return they retain the right to either kick them to the curb if they're no good or retain them for cheap if they are. The savings on Vettel and - this year - Ricciardo - probably funded not only their own junior careers but half a dozen other kids as well.
People often critic the amount of salary a sports person make over the course of his/her career, or even year. But why? Getting involved in sports is not easy at all. The mental preparation, sacrifice, etc, it takes helluva lot of dedication, discipline, and of course practice.
Most sports people have a career of less than 15 years, if they are lucky. One major injury, and you will fast become yesterday's news. I play sports, notably badminton and football. It's crazy the amount of time i take to recover from practice and games. The food intake, the sleep, the rest, protein intake...sometimes, it feels like there's no life.
Hence, i never gotten why their salaries get questioned. Not everyone can become a professional sports person. Especially in the case of racing drivers, you career or life, can be over in an instant.
You forgot the massive financial risk and investment as well. The overwhelming majority of kids who spend £250,000 a year to go karting end up with nothing except exam results that are about 3 grades lower than they would have been if they hadn't skipped school constantly and parents who are significantly poorer. For the one in a hundred who manage to make their living 100% in cars (which doesn't include a lot of say, the BTCC grid), most of them still aren't 'rich' by sports star standards even if they're comfortable by normal person standards.
It's like any high risk high reward profession. Yes, Brad Pitt earns millions but for every one of him there's a hundred people touring Shakespeare in schools or waitressing in LA.