How come open software like OpenOffice.org and supports proprietary formats like Microsoft DOC and MP3? AFAIK, the DOC format is not open and implementing the feature in OO.org required serious amounts of reverse engineering. So why isn't Microsoft (or any such proprietary players) making an issue out of this? In the same light, how come the GStreamer plugins are able to play proprietary formats?
the DOC format is not open
"Not open", here, actually means that it's controlled by Microsoft (who can change it at will) as opposed to decided upon by a committee (such as w3c for html). The format's specs is available for everyone to peek at -- if not by Microsoft, then by people who reverse engineered it.
So why isn't Microsoft (or any such proprietary players) making an issue out of this?
Why should they? They control the de facto standard of word processing documents. Allowing others to read their proprietary formats ensures things stay that way. This leads to more MS-Word sales. It additionally gives them a free hand to change the format whenever it fits them in order to introduce new features. Competition, by contrast, needs to rely on their own formats to add features, rendering their documents unusable except by other users of the same software.
First of all, proprietary does not mean closed. Microsoft has published it's binary formats (although MS Office was never 100% compatible with their own format). Current iterations of MS Office use .docx, which is Office Open XML, standardized by Ecma (as ECMA-376) and by ISO and IEC (as ISO/IEC 29500). Office Open XML is an open standard.
MP3 is just a abbreviation for MPEG Layer III. MPEG is also standardized, parts referring to MP3 areISO/IEC 11172-3 and ISO/IEC 13818-3.