Ive never been offered to become a protectorate, but I do find Thrace are very willing to become a my protectorate with little encouragement, i'm playing RTR as Macedonia and I offered the Thracian's 5000 denarii if they would become my protectorate, they had Tylis and the city immediately north to it left at the time and they accepted. I thought I could use them to fight the persistant rebel's that kept popping up west of the Black sea.

However upon the almost ceremonial yearly renewal of war with Dacia, Thrace sided with Dacia and I was forced to destroy Thrace.

Which was especially annoying as only 15 years earlier id pushed a Dacian invasion force out of there recently conquered Tylis in Thrace and then offered the city back to the Thracians for free, as there miltary was badly hurt I offered numerous subsidies to them so they could become a useful ally to me. Imagine the irritation when they decided to reward my mercy and kindness by siding with the war mongering Dacian's. Suffice to say Thrace was dealt with permanently and the war with Dacia is STILL going on 20 years later.