I have literally no clue as to why this means you need to say you're vegetarian... Particularly when it's based on a falsehood and you are essentially playing the odds and relying on the catering staff to share this falsehood... What happens if you get a waiter who has a GCSE or two?
If you want fish for breakfast, can you not just ask for fish, or is it a special hotel that requires its guests to be a bit mental?
Do you have to pretend to be something you are not for every request that you have or is it just food-based requests? Strange hotel....
Besides such randomness, if you wake up one morning in your special hotel and fancy beans on toast, or muesli and a boiled egg, or a cheese sandwich, why does this mean you need to pretend to be vegetarian? Every meal that does not contain meat does not need to be labeled 'vegetarian'.... It's just food innit...
Don't need to. Chose to, from experience, it works.
It doesn't matter what you think the definition is, it's understood that way in a lot of catering. So if you speak the same language as them, you're more likely to get what you want.
And if I get up in the morning and fancy a vegetarian breakfast rather than traditional English, it's not a problem. In that situation, to them as a customer, I'm vegetarian. They don't want or need to know the history, or why. Whether I was one the day before, or will be one the next day is of no concern at that mealtime.
Who's to say vegetarianism is a permanent state? Partial, or part-time, vegetarianism is just as valid an option. Restaurants certainly don't care beyond the meal that you're ordering.