I think that it is OK to use an open source project for commercial purposes, but to be on the safe side, make sure to read the license and the terms and conditions of the project that you want to use.

When it’s open source, the payment is not for the code but for another service you are enjoying. The community.

While the code is free as in free-food, the idea is to come because you need the food but you stay because the community is great.

In the what makes Wordpress, Ruby on Rails, Laravel great is not the code but the support you get from the community. When you run into trouble and need help, there is always someone knowledgeable enough to help you out. This person may be from the parent company.

Consider Discourse Forum, the code is open source and anyone can build their instance. But there is a pain in setting up that forum by yourself. So what you pay for is the added service to set up that code base on your website.

Sometimes open-source is used as a customer acquisition channel. The company offering the product give you access to the code knowing that once you get in they can market other products and features to you. At this point, you can’t decide to move your infrastructure to another service because it’s going to cost you more ( in terms of money, time, and technical knowledge ).

That said, the best CMS to create website is Wordpress. The community is great and there are lots of tutorials out there to help you get moving.

If you do create something with an open-source tool, except their is a special condition, you are allowed to profit out of it and own 100% of the profit. You may decide to give back to those who helped along the way.