I don’t think there is any mastodon code in truth social. My understanding is the code is based on Soapbox and Pleroma which are compatible with the ActivityPub protocol and some of Mastodon’s additions to it, but it’s not a fork.

Even if there were, there’s nothing wrong with reusing open source code in new and different ways, that’s the entire point, it’s not to give you software you don’t have to pay for, it’s about creating a common set of tools to build together in the commons.