Why are these tests passing?

I have this function:

let removePresentation = function(presentationName, callback) {
  let rimraf = require('rimraf');

  callback();
  callback();
  callback();

  if(!presentationName || !presentationName.trim()) {
    callback();
    return;
  }

  presentationName = presentationName.replace('.zip', '');

  rimraf('./presentations/' + presentationName, function(err) {
    if(err) {
      console.log(err);
    }
    callback();
  });
};

exports.removePresentation = removePresentation;

and I am trying to test it with the following:

var chai = require('chai'),
expect = require('chai').expect,
sinonChai = require('sinon-chai'),
sinon = require('sinon'),
mock = require('mock-require');

chai.use(sinonChai);

describe('removePresentation', function() {

  var sandbox;
  var callback;
  var rimrafSpy;

  beforeEach(function() {
    sandbox = sinon.sandbox.create();
    mock('../business/communications_business', {});

    rimrafSpy = sinon.spy();
    callback = sinon.spy();

    mock('rimraf', rimrafSpy);
  });

  afterEach(function() {
    sandbox.restore();
  });

  it('should call rimraf if presentation name is valid', function(done) {
    let RoomStateBusiness = require('../business/roomstate_business');

    RoomStateBusiness.removePresentation('name.zip', callback);

    expect(rimrafSpy).to.have.been.calledWith('./presentations/name');
    expect(callback).to.have.been.called.once;
    done();
  });

  it('should not call rimraf if presentation name is null', function(done) {
    let RoomStateBusiness = require('../business/roomstate_business');

    RoomStateBusiness.removePresentation(null, callback);

    expect(rimrafSpy).not.to.have.been.called;
    expect(callback).to.have.been.called.once;
    done();
  });

  it('should not call rimraf if presentation name is whitespace', function(done) {
    let RoomStateBusiness = require('../business/roomstate_business');

    RoomStateBusiness.removePresentation('      ', callback);

    expect(rimrafSpy).not.to.have.been.called;
    expect(callback).to.have.been.called.once;
    done();
  });

  it('should not call rimraf if presentation name is empty string', function(done) {
    let RoomStateBusiness = require('../business/roomstate_business');

    RoomStateBusiness.removePresentation('', callback);

    expect(rimrafSpy).not.to.have.been.called;
    expect(callback).to.have.been.called.once;
    done();
  });

});

Even though I am clearly calling callback() multiple times (whilst testing only), expect(callback).to.have.been.called.once; is always asserting to true. I have checked on the Chai api that that expects the call to be exactly once, although it is always passing no matter how many times I call callback(). What am I doing wrong?

from Why are these tests passing?