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You're not wrong, but the License used is critical here. If the code is indeed Open Source, with an OSS license, then you can use it as-is, or just learn from it and write-your-own. You might even fold it as-is into your app. Keep the code, but remove the dependence. Free Software on the other hand is a different animal. The GPL et al is viral. Doing any of the above with GPL software has consequences. Even learning…
Free software licenses and open source licenses are almost the same set, and the difference between the two is not copyleft: the GPL is OSI-approved. The difference is in the spirit, focus, philosophy: free software is concerned about user rights. And if you care about user rights, use a copyleft license to prevent someone from building a proprietary derivative, even if you are not turning it into your day job. Espec…
If all you want is to offer "here's some code, knock yourself out" then a license like MIT and BSD is generally more useful to more people.