Very often when talking about vegetarian food, in restaurants or on labels for food, eggs are considered as suitable for vegetarians. There are some labels "lacto-vegetarian" but not so common (at least everywhere I went so far).
I don't understand any more how the eggs of any animal could be considered vegetarian. Where is that coming from and why has this idea stuck everywhere?
In Europe and the Americas people who consider themselves vegetarians in general only avoid foods that are the direct result of an animal's death, such as meat.
This does not technically apply to eggs, which are produced by hens who are not killed and in theory may be unharmed in the process. Ovulation happens in healthy adult animals without interference. Eggs may therefore be considered vegetarian according to the Euro/American usage of the term.
As an aside, the UK Vegetarian Society will only approve products that contain eggs if the eggs are "free-range" because this is believed to provide a better quality of life for the animals.
Not all vegetarians do eat eggs. It's common that western vegetarians eat eggs but asian vegetarians do not. Airlines cater for this with their meals, offering both western (VGML) and asian (AVML) vegetarian meals.
So whether eggs are or aren't vegetarian is cultural rather than universal and if you lived in India for instance you might well be of the opinion that eggs are not commonly allowed in vegetarian food.
Eggs that are sold commercially are unfertilised. Hens do not need a rooster to lay an egg. So since the chicken that layed the egg lives and the egg itself can never become a chicken it is traditionally allowed as part of a western vegetarian diet.
As @Scimonster mentioned in the comment, this has to do with the ambiguous definition of the word 'vegetarian'. Depending on who you ask it can either mean 'does not eat meat or fish' (as in ovo-lacto-vegetarian) or 'does not eat animal products' (as in vegan). Both are valid definitions, so calling eggs vegetarian is not wrong.
To back this up, here are some definitions of the word 'vegetarian':
Merriam-Webster (I refer to the definition of the adjective, but the one for the noun is similar, number 2 is the one of interest here)
1 : of, relating to, or suitable for vegetarians
2 : not containing meat : consisting wholly of vegetables, fruits, grains, nuts, and sometimes eggs or dairy products
<a vegetarian diet> <vegetarian lunch>
Oxford Dictionary (This one relates to the noun. number 1 is the one of interest here)
1 : a person who does not eat meat : someone whose diet consists wholly of vegetables, fruits, grains, nuts, and sometimes eggs or dairy products
2 : herbivore
A vegetarian is someone who lives on a diet of grains, pulses, legumes, nuts, seeds, vegetables, fruits, fungi, algae, yeast and/or some other non-animal-based foods (e.g. salt) with, or without, dairy products, honey and/or eggs. A vegetarian does not eat foods that consist of, or have been produced with the aid of products consisting of or created from, any part of the body of a living or dead animal. This includes meat, poultry, fish, shellfish*, insects, by-products of slaughter** or any food made with processing aids created from these.
As you can see all of these state allow for either variation, and the list is far from complete, if you look up the definition for vegetarian, you will mostly find that it has this ambiguity.
Den, have you ever had chickens? They do their thing and lay eggs every day, even when there isn't a rooster around i.e. they are not fertilised. This is also the case with commercial eggs.
For me, it isn't a problem as I am not harming an animal by consuming eggs.
Of course there is another side to this and that is the treatment of chickens in commercial egg production, where they are nothing more than egg laying machines. If you trust the claims of 'free range' egg producers that might satisfy you.
Many years ago there was an egg farm in my street where the chickens were squashed 3 to a cage. It was truly awful. The farmer (and neighbour) offered us free eggs any time we wanted them. I never took one.
I guess it's better not to cling to some strict definition, since it's partly based on very personal feelings. Try to look at it this way: vegetarianism or veganism are just shades of avoiding suffering. It's just a question of how consequent one's living to it. If eggs are suitable for vegetarians is the same like asking, if honey is suitable for vegans. So, to answer your question: Not being responsible for the death of an animal is, for most people, a very big step. And most people stop here, they don't distinguish between killing and (ab)using an animal.
And some more thought to eggs: Naturally, a hen is laying eggs to breed. If one or more eggs are taken away by some predators, it lays some more eggs to complete her clutch again. Over hundreds of years forcing hens to endlessly completing their clutch to no avail lead to our modern day hens. So I don't believe hens are completely happy laying eggs and so I don't take their eggs away.
