Mormons believe there is a waiting period between death and the resurrection/final judgement during which waiting period the spirits of the dead are in various states of waiting depending on the state of their soul at death.

Mormons believe that the gospel will be preached to all who did not accept it during this life, whether out of rebelion, ignorance, or anything in between. Baptisms are performed on behalf of all deceased, who, according to mormon belief, can choose to accept it or not. Even the mormons believe that the baptism is meaningless unless the deceased expressly accepts it.

Of course anyone can choose to be offended at this practice if they wish. But it is entirely unintrusive and done only out of sincere love for all people. You would actually have to go out of your way to even know if it had happened.

If people feel it is condescending, well it is no more condescending than someone trying to share their faith with you in an unintrusive manner. I've had people try to share their faith with me, I find it endearing, even if the tacit implication is that they think my beliefs are wrong, heretical, stupid, etc. I don't see how it is really effing rude.