Developers support copyleft because it ensures that no proprietary interests will be able to benefit from the time and effort they graciously donate to the community. This prevents companies from taking developers' free code, adding functionality, and denying the source changes to the very community that enabled their work in the first place.
Developers support copyleft also because it reduces code forking.
Despite some rather interesting arguments over the years on the kernel mailing list, the development of Linux has had few minor and short-lived branches away from the primary code base. Finally, developers support copyleft because it works. Consumers, whether they know it or not, also benefit from copyleft. This ensures the proliferation of free software, the commoditization of end-user applications, and the consequential migration of proprietary interests toward the edge of software development and away from the typical consumer. The stability, extensibility, and standards compliance ensured by the free software model will make this a benefit for all consumers.