Is it legal to develop iOS apps on Windows?
Yes

…it's legal to develop iOS apps on hardware or software other than Macs and OS X respectively as it does not violate the Registered Apple Developer Agreement:


https://developer.apple.com/programs/terms/registered_apple_developer_20100301.pdf


You can develop iOS apps on any platform that you can, using a variety of software available. The legal problems come from running OS X on unsupported hardware, or running pre-release software without being a developer, etc.