Why weak daeef Ahadith in Fazail e Amaal Is it right or wrong

Nevertheless there exists an overwhelming majority of Islamic Scholars (from Salaf & Khalaf) who not only permitted its usage but their publications contained these (i.e. Weak Narrations) in abundance. 

Ameerul-Muhadetheen Imam Bukhari (RA)

was the Master of Hadeeth and clearly knew the difference between Authentic (Saheeh) and Weak (Da’ef) Ahadeeth

YET he chose to use weak Ahadith in a book of Fadhail (virtues) of Amaal;

His famous book on fazail e amaal/virtues Al Adab Al Mufrad is read, taught and practised by the Ummah for generations with the full knowledge that it contains Weak (Da’ef) Ahadeeth.

some of the chapters in the book contain no Authentic narrations at all.

Allamah Shaykh Abdul-Fattah Abu-Ghuddah has discussed the chains of transmissions of Adab Al-Mufrad in detail in his commentary “Fadhlullah As-Samad”

2) Even in Saheeh Bukhari while discussing the Hadeeth, “Be in this world as if you were a stranger or a traveler.” Hafidh Ibnul Hajr Asqalani (RA) comments in Fathal Bari, “Muhammad Ibn Abdul-Rahman Tafawi is Munfarid in the narration of this Hadeeth, perhaps Imam Bukhari (RA) has shown leniency in the matter because this Hadeeth is about Al-Targheeb Wa Al-Tarheeb.
Source: Fathul-Bari Kitabur-Raqaiq

The full Hadeeth is as follows:

Narrated Mujahid (RA) : 'Abdullah bin 'Umar (RA) said, "Allah's Apostle (Sallaho Alaihe Wassallam) took hold of my shoulder and said, 'Be in this world as if you were a stranger or a traveler."

So in Fazaile Amaal shaikhul hadith Maulana Zakaria has applied same principle. 

Those who are criticising Fzail e Amaal/shaikh zakaria should undersatnd the things in correct perspective and should be fearful with Allah.
Allah knows the best.