What I mean to ask is that, can I legally use woocommerce free theme on a commercial ASP.NET website?
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You would need to see the licensing of the theme that you would like to use. When you pay for a theme, you are actually paying for the license. There are many licenses that allow the content to be free to use but that does not mean that you could use their source code for free.
Not every thing which is free means that it's source code is available for everyone else to copy.
In case you happen to look upon themes whose source code is not licensed, you can use it, but just be on the safe side and inform the developer about it before implementing the source code.