iserith
Magic Wordsmith
While the DM is within their rights to do this, it is not possibly the best fit for D&D. Often mentioned as part of the conventional wisdom in "how to DM" articles is "being arbitrary or inconsistent breeds discontent". If this was an isolated happening just to shake things up, its not too bad. If there's no rhyme or reason to when and where this is happening then it will get frustrating for players very quickly. If it the up-front house rule that things work this way (with or without the PC having a choice) then it could be fine.
tl;dr: Being arbitrary or inconsistent breeds discontent. Meh as a random ruling, workable as a house rule.
Is it really arbitrary though? It's based on a method that exists in the game system which is also supported by "How to Play," and reasonably fits the context of the fiction. I don't believe it is based on random choice or personal whim. Thus I think it fails the test of being considered arbitrary. There is also nothing in the example that suggests the DM is being inconsistent - this could be how the DM normally rules.
Of course, if it's not one's cup of tea, that's fine. I just don't think it could be said to be arbitrary or inconsistent.