In the original Loremasters Guide there was a section that asserted pretty strongly that the GM shouldn’t change off the default 14 unless he had a very strong reason for doing so, and that changing the TN was making a important statement about the relevance of the task.

The phrasing and repetition of this guidance suggests to me that the player is entitled to not be surprised by the TN, so it is the GM’s responsibility to make the fiction crystal clear when the TN is going to be something other than 14.

If a player is willing to go forward based on the fictional description, but then balks at the TN assigned, that suggests that the GM faltered in making the fiction carry the message of how difficult the roll was going to be.

I don’t remember an explicit direction to allow the player to back out; but the logic above is what suggests to me that was the design intent.