Let's bear in mind that someone is being locked inside a building for a while, just because he once ran a website that became unpopular with the authorities. Debating whether or not he should be fed a vegan diet on the inside is prematurely surrendering the debate that questions whether he should be losing the freedom to control his own diet for that supposed offense.
It is not a binary choice between just loss of freedom and loss of freedom plus compromise of one's dietary practices.
It seems odd that we would be assigning the same punishment regimes to people so pacifist that they won't even eat plankton as we assign to people who destroy human lives with nary an inkling of remorse.
Veganism is something I can understand and respect. The morality of state justice, on the other hand, is far less defensible when you look past its well-groomed public image. Depending on your locale, the prisoners in your nearest prison may be treated with less respect than the animals in your closest slaughterhouse. In that light, making morality-based appeals to the prison kitchen seems almost pointless.