It's not only correct, it's actually very common to use "are a" in English. Here are a few examples:
These criminals are a threat to society
I married my girlfriend, because marriages are a good way to show people you love them
The gifts he gave you are a good reason to think he likes you!
These example work because every criminal I was talking about was a threat to society, every marriage is a good way to show people you love them, and all the gifts together are a good reason for the girl to think the boy likes her. When we talk about things as a collection serving a specific purpose, "are a" is a very common construct.