Salam Sardar Miyan
The real question is why is the Imam saying such a thing? Maybe because the attahiyat and other recitations the Imam is saying we can recite on our own( according to him) is constant for all, if prayer is offered in the traditional manner. Traditionally we all recite the same thing on those stages so it is not going to be any different than what the Imam is reciting.
But, when it comes to reciting the second surah after the recitation of surah fatiha, we have a lot of choices, anyone can choose to recite any sura, no matter how long or how short. But if that is allowed to happen then everyone will not be in synch while praying in congregation.Someone will finish prayer an hour before another, depending on the length of the sura he wishes to recite after surah fatiha.
Because of the ease of synching prayer with each other and the imam, they say we cannot recite anything we want at this stage. This will lead to the very real possibility of someone prostrating for fourth rakat and another for second rakat at the same time, for a four rakat salat. This has no other reason in my opinion.
However, if we choose to recite the same surah along with the Imam, then what is the problem? This is not going to be possible all the time because traditionally most muslims offer some prayer silently. Which the Quran opposes. If you r not aware what surah the imam is reciting u cannot recite along with him, atleast during these so called silent prayers. That is why they justified it in their minds, that it doesn't matter, just follow the robotic movements of the imam. U will be ok during all prayers, silent or not silent. Doesn't matter what the imam is reciting. Just follow the Imam's lead! All this is done to justify what they are doing. None of this is mentioned in Quran.
But the most important question is, why is the Imam saying such a thing? What is his source? When the Quran is absolutely silent over what we should recite while praying and it is completely our choice. The Imam is only doing this for practical reasons, this has no Quranic sanction.