(عَفَا اللَّهُ عَنكَ…) (May Allah pardon you [Oh Muhammad],): “Pardon”; here indicates that there is a matter that has been forgiven. The Prophet (saws), was compassionate and merciful towards his people. So, when Allah (swt) says: May Allah pardon you‘ it means that there is no trace of the issue left. The Prophet (saws) granted them permission out of his compassion and mercy towards them. This is evident from the fact that Allah (swt) supported the Prophet’s stance when He said:

 لَوْ خَرَجُوا فِيكُم مَّا زَادُوكُمْ إِلَّا خَبَالًا وَلَأَوْضَعُوا خِلَالَكُمْ يَبْغُونَكُمُ الْفِتْنَةَ وَفِيكُمْ سَمَّاعُونَ لَهُمْ ۗ وَاللَّهُ عَلِيمٌ بِالظَّالِمِينَ 

(Had they gone forth with you, they would not have increased you except in confusion, and they would have been active among you, seeking [to cause] you fitnah. And among you are avid listeners to them. And Allah is Knowing of the wrongdoers) (At-Tawbah: 47)

This means that their participation would have added nothing but hindrance. However, Allah (swt) legislates, and the Prophet (saws) was granted mercy and a broad heart towards people. He (saws) allowed some of them not to participate, and Allah (swt) stated His disapproval for that, clarifying that He had forgiven his Prophet (saws) when he gave some people permission not to go for the battle. This is an honor for the Messenger of Allah (saws). Blaming in the Quran for the noble Prophet is an elevation of his status, unlike the reproaching directed to other humans. The Prophet (saws) did not do this out of inattention, but rather out of his compassion, mercy, and efforts at guiding his people.