MacRuby is open source and actively developed on Github. Lock in isn't Apple's modus operandi. If Apple didn't want to people to use Ruby they wouldn't financially back it and add Ruby language support in XCode.
Also it is not like Objective-C doesn't work other platforms. Linux has two working Objective-C compilers, Clang and gcc. And guess who developed the Objective-C compiler support? Apple. If they are trying to lock Objective-C devs into their platform they are doing a very poor job of it.
Ruby is a hugely popular language amongst the Mac-using hipster developer community so it is only natural for Apple to back it.