Why no parenthesis when using event Handler, but yes for other function calls?

Regarding the “Random Color” Project for React Part I

At the end, there is a <Button> described as such: <Button onClick={this.handleClick} light={this.isLight()} />

When <Button> uses this.isLight() it requires parenthesis, however the event handler does not use parens, it just has this.handleClick.

It is my understanding that this.functionName() is a call to the function and just this.functionName is a reference to the object for the function (at least in Python).

But I feel like it should be the same for both. Why is the syntax different here?

Full Code Below

Random.js

var React = require('react');
var ReactDOM = require('react-dom');
var Button = require('./Button');

var Random = React.createClass({
  getInitialState: function () {
    return {
      color: [0, 0, 0]
    };
  },
  
  handleClick: function () {
    this.setState({
      color: this.chooseColor()
    })
  },
  
  componentDidMount: function () {
    this.applyColor();
  },

  componentDidUpdate: function (prevProps, prevState) {
    this.applyColor();
  },

  formatColor: function (ary) {
    return 'rgb(' + ary.join(', ') + ')';
  },

  isLight: function () {
    var rgb = this.state.color;
    return rgb.reduce(function(a,b){ return a+b; }) < 127 * 3;
  },

  applyColor: function () {
    var color = this.formatColor(this.state.color);
    document.body.style.background = color;
  },

  chooseColor: function () {
    for (var i = 0, random = []; i < 3; i++) {
      random.push(Math.floor(Math.random()*256));
    }
    return random; 
  },

  render: function () {
    return (
      <div>
        <h1 className={this.isLight() ? 'white' : 'black'}>
					Your color is {this.formatColor(this.state.color)}!
        </h1>
        <Button	onClick={this.handleClick} light={this.isLight()} />
      </div>
    );
  }
});

ReactDOM.render(
  <Random />, 
  document.getElementById('app')
);

Button.js

import React from 'react';

export class Button extends React.Component {
	render() {
		return (
			<button 
				className={ this.props.light ? 'light-button' : 'dark-button' } onClick={this.props.onClick}>
				Refresh
			</button>
		);
	}
}