IMHO, If someone wants to use a proprietary product with FOSS, so be it,
denying this right to him/her would indeed be against the FOSS philosophy.
There is no single thing called FOSS philosophy, there is *Free software
philosophy* and *Open source philosophy* which are very different, though
the technical definitions sound almost the same. If you really want to
understand the difference, read this quite long story [1].