Hello, Al-Shadly, and welcome to the Grammar Exchange!
The collocation is "critic of" when the preposition introduces the issue being criticized.
I have found that in Canada the members of the opposition shadow cabinet are known as critics, with "for" introducing the area that those parliamentary members deal with. "critic of" would mean that they are against, not that they devote their time to those aspects or fields. We thus find examples like this one:
- Mable Elmore is a Canadian politician who was elected as a Member of the Legislative Assembly of British Columbia in the 2009 provincial election. A member of the BC New Democratic Party, she was elected to represent the riding of Vancouver-Kensington. In the 39th Parliament, with her party forming the official opposition, Elmore was initially the deputy critic for child care and early learning. She served on the Select Standing Committee on Children and Youth. She was then also appointed as critic for multiculturalism and immigration. Currently, she is the Critic for ICBC and the Deputy Critic for Finance. (Source: http://www.self.gutenberg.org/...les/eng/Mable_Elmore)
"for" can also be used to introduce the entity for which one acts as a critic, as in this example from the Longman Dictionary:
- He became the chief music critic for the Herald Tribune.
And why it was not " ..as a critic.." ?
As in these cases "critic" is a position, it takes either the zero or the definite article.