Did the Imam Cut Grass Sinfully in the Masjid?

The Shubha:

(1322) 235 - موسى بن القاسم عن جميل بن دراج عن أبي عبد الله عليه السلام قال: رآني علي بن الحسين عليه السلام وانا اقلع الحشيش من حول الفساطيط بمنى فقال: يا بني ان هذا لا يقلع.

Musa b. Qasim from Jamil b. Darraj from Imam Sadiq (a), who said: Ali b. Husayn (a) saw me uprooting the grass around the tents in Mina, and he said "Oh son, these should not be uprooted." (Tahdhib al Ahkam)

"Uprooting grass or cutting down trees in the Haram is forbidden. However, there is no objection to grass being damaged in the course of ordinary walk. So too is letting animals feed on the grass. It is advisable, though, to avoid cutting grass to feed them." (Sistani.org)

According to orthodox Imami theology, the Imam is infallible from birth:

Yet, even before they attained the age of religious responsibility, they were not subject to defects and ignorance since they were of the pattern of Jesus and John, peace be upon them, in that they were endowed with perfection though they were still children, and had not attained the age of discretion. And, in fact, this is a tenable proposition, one which admits of rational proof. (Tashih al I'tiqadat)

So if the Imam is infallible from birth, then he cannot do any sins, even before he is baligh. But here, the Imam is uprooting grass in Masjid al Haram, which is a sin. Hence, infallibility is disproven from this narration. 

The Response:

This narration is not authentic. Rather it is munqata', meaning that a narrator seems to have been omitted from the chain. As Sheikh Hasan Sahib al Ma'alim says in Muntaqa al Juman:

وهذا الحديث منقطع الاسناد لأن موسى بن القاسم يروي في الأسانيد المتكررة عن جميل بن دراج بواسطة أو ثنتين ورعاية الطبقات قاضية أيضا بثبوت أصل الواسطة وفي جملة من يتوسط بينهما في الطرق التي أشرنا إليها إبراهيم النخعي، وهو مجهول والعلامة مشى على طريقه في الأخذ بظاهر السند والاعراض عن إنعام النظر فجعل الحديث في المنتهى من الصحيح

And this Hadith is cut off in its chain, and this is because in the chains from Musa b. Qasim to Jamil b. Darraj, he repeatedly narrates via an intermediate or two. And examining the tabaqat (of the narrators) also establishes the origin of an intermediate. And among the people who intermediate between the two of them (Musa b. Qasim and Jamil b. Darraj) in the chains that we have shown is Ibrahim b. Nakha'i, who is majhul. And the 'ulama have followed upon the chain in taking the apparent from the chain, and avoided benefitting from investigating, and so [this is why] they've made this hadith Sahih. (Muntaqa al Juman) 

Because this narration is not authentic, it is not an evidence against us.