Is it safe to use painted unmachine rims with rim brakes?
For your specific bike, the Masi Riser, there is nothing to worry about. 

Those rims are from BrevM. From the review of some brevm wheels on urban velo, it says that the rims were machined before annodizing.  So, the annodizing masks the brake surface, but it should work fine.  Even if they were not machined ahead of time, assuming good quality control, there really shouldn't be an issue unless they were made for disc specific mountain bike or cyclocross applictions. 

So long as your rims are not disc specific, but not machined, then you should be good, and it looks like that's not the case here.  Many times fixed gear bikes are sold without a machined brake surface for the seamless look, especially for those who prefer to ride brakeless.