Interesting twist to the language.
The poster wanted to not have to write he/she is a traitor, changed he/she to they, and then had to use are.
It is considered acceptable in this context.
The issue is related to gender neutrality.
Please have a look at the gender neutral pronoun singular they:
Since at least the 14th century, they (including derivatives and inflected forms, such as them, their, theirs, themselves, and themself) has been used, with varying degrees of general acceptance, to refer to a singular antecedent. This usage is often called the singular they. Today, it is unexceptional and often not regarded as incorrect, especially in informal language.Wikipedia