Hey there u/d and Aude!
Good observation -Aude. I'm also inclined to think that the anniversary celebrations slowly "crept" in over the years and then became so popular that the GB or its advisers realised that to forbid the such anniversaries would create too many problems.
Let's face it: There are so few opportunities for the JWs to celebrate without guilt that a bit of lee-way was allowed - but policy probably developed "after the fact". To get around this, the WT simply says smug things like wedding anniversaries are not for everyone or it's a conscience issue. But again: Why not say that about birthdays as well?
I had the same sort of question about "pagan" wedding rings. The JWs would say that even to have pagan symbols in your house would be to inadvertently "invite" Satan into your house. I remember having a picture of a horse-shoe on a calendar in the 1970s. A pioneer brother who visited said that he couldn't have this pagan symbol of good luck in his house because of the risk of satanic influence. Yet he sported a golden-band wedding ring whose pagan connections he could safely ignore!
steve2