future simple

scenario: Rubens Barrichello is leaving for BAR Honda next season....

so, this has clearly been arranged (contracts have been signed etc), so we should use present simple continuous or going to.
yet, i just read:

"Barrichello, who will join BAR for next season, was working on a programme centred on set-up and tyres for that race, while Massa, a former Ferrari test driver, spent his time familiarising himself with the F2005 and fine tuning the car."

so, my question is...why do they use "will join" and not "is joining / going to join"?

are they all valid?

Thanks!

Tania