我抄了下面的答案this question.
由于WordPress是GPL代码，所有公开发布的代码也必须以GPL或GPL兼容的方式获得许可。
GPL声明您不能对代码收费，但可以对分发收费。因此，当你购买gravity表单时，你不是在为插件付费，而是在下载和获取插件。
Once you have the plugin it is perfectly legal to burn it to a CD and
mail it to 20,000 people free of charge. You won\'t get the support or
updates ( unless you pay them again, or you have a support contract ),
and it\'d be a pretty nasty thing to do ( they have a business! ), but
it\'s perfectly legal.
如果以上是真的，为什么许多插件作者通过“1个站点许可证”、“2个站点许可证”、“无限站点许可证”销售插件。
根据上述答案even though I buy "1 site Licence", I should able to use it on unlimited sites as well as my clients sites legally? 是否正确？
