I don't see why TAM can't keep agreements with Star partners, if they make economic sense then they should keep them whilst also codesharing with Oneworld airlines.
Oneworld seem very liberal with cross-alliance codeshares, and cross alliance codeshares are very common in all alliances. Just because airlines belong to the same alliance doesn't mean they are obligated to coeshare with every alliance member and not co-operate with any airlines outside from the alliance.
Qantas and AF are good examples, QF would rather put their code on Air France flights rather than a codeshare with Cathay Pacific. In Star SQ doesn't co-operate with Thai, United, Aegean, Continental, Adria etc....