I forgot where I heard this, because it has been many years ago:
"One becomes a monk, not just to be vegetarian"
You're not forced not to have meat, but if you decide to devote yourself and stay in a temple, meat isn't what is served daily.
Other words, what if you sneaked out and have meat and get caught?
At certain temple, they don't punish you by expelling you, but most would, if not expel, warn you. Or you would be "punished" and be isolated / assigned to certain task where you carry out the work and at the same time "reflect".
As to why some of the monks still eat meat, smoke, drink alcohol etc:
I can't say all; but what I have seen is when I was backpacking in
Thailand: In Thailand, when one graduates from high-school, students
are required to either take part in military or in a temple becoming a
monk for around 6 months (exact time I have forgotten). Many chose to,
as was told to me, to become a monk at a temple.
So, those who send themselves to the temple after high school are not
really those who really would like to follow Buddhism. For what I have
seen there, some of them smoke, belong to a gang, were involved in
unlawful activities, etc etc.
Another reason I was told is: some know what they do are bad, those
who take part in gangs, etc etc, so out of this reason, they "hope" to
repent and feel "protected". But afterward, they mostly remain where
they were before they became a monk after high school.
The above is only what I have seen then and was told to me by the
So, what it is about becoming a vegetarian being a monk / Buddhist?
You do so gradually, through practice and reading (addendum: please don't be affixed on words and their meaning; they are meant to be guides / guidance).
There is a saying:
"refrain the mouth from meals (meat), but fail to do so at heart (you
still crave for it; you have to force yourself not to have it)"
That's not what this is about; you don't have to force yourself in / into doing anything in Buddhism.
Through practice and observance and patience, when one has attained eventually understanding, you would stop eating meat or other' being's flesh. It is meant to be a normal transition, if one finally understand "the way" of Buddhism, which involves time, patience, observance, reading / meditating / reflecting.
It comes to you normally and will so eventually, though the practices mentioned above, when one finally understand it.
I was told the following saying many years ago, which I still remember to this day:
"In Buddhism, it is about Destiny, if you so approach it in your
lifetime, it is Destiny; you choose to believe it and follow it, it is
Destiny; and if you choose to leave it and betray it, no-one would
send you out the door, for you are welcome to depart, because no-one
has held you in it at the first place - it is also Destiny."