If tall trees was something you discussed with the contractor, maybe one of his employees got it wrong and he decided not to notice. I think the trees will grow upwards by way of a secondary leader. The trunk will be offset, though.
I would contact the landscaper and tell him it was your understanding that trees would be allowed to grow tall, especially after your discussion about eventual height. Its too bad, though, that you didn't pursue this immediately - 6 months after planting is going to be tough to convince him to do anything. It sounds now like you simply changed your mind.
If you do decide he won't do anything, replant the extra arborvitae sooner rather than later.
