In genealogy we HAVE to be aware of privacy and data protection (even though we do it without thinking, and may not call it that).
Many of our primary sources of reference (BDM certificates, Parish Registers etc) are all copyright so scans should not be published.
The {UK} National Archives provides guidance on the copying of BDM certificates in the UK, which contrary to my original opinion, are not copyright - although copying of BDM certificates of living individuals is not permitted.
However, as @Justin808 has observed, Gravestones are public monuments.
On the other hand, I have personal issues about photographing random stones, so restrict myself to known family members - in which case the answer to the question is "because they're my 2nd cousin 3 times removed - that means we're related. I'm Andrew, how do you do!"