Genealogical courtesy

If you have found information relevant to your family on the internet that another genealogist has researched and kindly made freely available, then

a) it's only right to acknowledge them as a source if you are republishing their information, and

b) why not email them to say hi, I see we have common ancestors, nice to meet you and thank you for making that information available.

On the odd occasion (for example, just now) when I have come across someone republishing my own family information (some of which is so specific I know it can only be traced back to me), it does make me wonder why that person never thought to email me. I don't have a problem with the information being out there, after all I did make it available in the first place, but it seems just a tad impolite, does it not?