Why does HTTP service require @Injectable decorator and a custom service does not in Angular 2?

Why is it, that I need to include @Injectable when doing DI using http, but I don't need it when doing DI using a custom service I created?

For example:

//no @Injectable() here
export class Test {
constructor(private customService: CustomService){}
}

/*-------------*/

@Injectable() // <== required here
export class HttpTest {
constructor(private http: Http){}
}

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