Do international scientific conferences in us have permission to send invitation letter ?

i am a student of university and i am from iran.about one month ago i submitted an article in a us international conference and they have accepted my article and asked me to attend there as a speaker.but after i have asked them to send me an invitation letter for us visa,they refused and said it is not possible for them to send invitation letter and said to apply for a tourist visa and say nothing about the conference in the embassy,what is that for?is it a new rule?