Military out of state Trade-in sales tax?

I read that buying a car in a state with no sales tax you would still pay the taxes of where it gets registered so my confusion is this.... I am currently in the military home of record and legal residency is Texas where you dont pay sales tax on the value of your trade-in amount and only on the difference to the new car. I am currently stationed in california and they are saying i would have to pay california sales tax on the whole price of the new vehicle no matter my trade-in value even if i get it registered in texas and i dont think that is right.. Why wouldnt i used texas sales tax and regulations since im going to get it registered and titled there??? If i have to pay cali sales tax cause im buying it there then why cant i just not pay sales tax buying it in a sales tax free state?