If you give people your software under the MIT License, they can give it to anyone they want to.
"Permission is hereby granted, free of charge, to any person obtaining a copy of this software and associated documentation files (the "Software"), to deal in the Software without restriction, including without limitation the rights to use, copy, modify, merge, publish, distribute, sublicense, and/or sell copies of the Software, and to permit persons to whom the Software is furnished to do so"
You said "only people who pay for my software get access to it". I was pointing out that anyone your customers choose to share your software with get access to it.
Sorry, I let out the words "from me" because I thought it was implied. I don't share this open source software. I sell it.
freshhawk listed two categories: 1) a "dump of useful stuff" with "no responsibility to maintain it" by people who are "nice enough to share." I don't fit in that because a) I include a year of support in the sale, and b) I don't share it, I sell it.
2) "Then there are actual projects. A person or group saying 'Here is a problem we all share. ... I will do some management work on this in return for your contributions and my name being prominent'". I don't expect trade in kind, I expect money. Just like proprietary software vendors (especially those who distribute sources).
"Permission is hereby granted, free of charge, to any person obtaining a copy of this software and associated documentation files (the "Software"), to deal in the Software without restriction, including without limitation the rights to use, copy, modify, merge, publish, distribute, sublicense, and/or sell copies of the Software, and to permit persons to whom the Software is furnished to do so"
http://opensource.org/licenses/MIT