shocked - Need help!

Something happened. My bf's mom came over to visit. When she walked into the house, she gave my bf (her son) a hug and kissed him on his lips! It was a quick kiss. But I was shocked! They did it so naturally like it's totally normal! I have never seen them kissing each other on the lips before!

I asked my bf what's going on. He said it's tradition. His mom always kiss them (including his brothers) like that.

I noticed that my bf does not want to show too much passion toward me when his mom is around. I asked him why and he said that our relationship should be private.

My bf's dad passed away long time ago. His mom never remarry and does not have a bf right now. She is almost 70.

But is that kind of behavior normal?