Yes no doubt that Sajdah (prostration) should be only for Allah (swt) none other than Him. But when we go to the Holy shrine of Aimmah (a.s) we do sajdah in front of their Zareeh or at the entrance with this Niyyat that O’ Allah thanks for giving us this opportunity to visit and show our tributes to our holy Imams if this is the Intention then there is no harm in doing such type of Sajdah. But if any one for God forsake does Sajdah to any of Imam’s Zareeh for Imam then this is totally invalid and Shirk.
Hence if doing sajdah at the shrine of Imam even with the intention of that, this is my Sajdah to Allah only causes any misconceptions or misunderstanding amongst the other sects of Islam that we Shia do sajdah of our Imams then it’s better to avoid doing so.