This is sadly the official shia position that sunnis are muslims in this world but will not be in hereafter due to their rejection of imamah. If this is the reason then I strongly believe that the blame will be on those 12 imams if they really were divine agents. Because they themselves never did any public announcement so that masses may know who is the divinely appointed imam/guide whom they are supposed to follow. Whats the point of imamah (whose objective is t o lead and guide the ummah) if masses dont know who the imams are.
There is no logic in this.
This is not the official Shia position. If you are not aware, i have grandfather who was Sunni who passed away. Many of my relatives are actually Sunni. Our position is that we will pray behind you, it is permissible to marry you, eat your slaughtered meat, and you are considered Muslims. However, in the hereafter anyone who knowingly rejected the true path, not out of confusion or having a distorted picture of it, or perhaps not in the right environment to truly ascertain the truth, but rather rejected it out of arrogance knowing it to be true, or let their arrogance or desires refrain them from accepting it, will be judged in that way.
Our position and tolerance towards Sunni's is far greater than what you find among the Salafi-Athari's and others, who say:
1. Don't pray behind a Shia. 2. Don't marry them 3. Any Shia who believes in official Shia beliefs is not a Muslim.
At least with us, we let Allah be the judge in the day of judgement, and many Sunnis may enter Jannah, if Allah so wills.
Put Sunni's aside, we believe it is possible for non-muslims who never received the truth, or received it in a distorted form, rather than ignoring it our of arrogance or desires, or were influenced by it presented to them in an incorrect way and were convinced it was not correct due to that, to enter Jannah.