Why Don't All Ismailis Eat Only Halal Food?
The recent Imāms have not said anything about halal food. Mawlana Sultan Muhammad Shah said that when you give dasond, your earnings and food become halal.
In fact halal is even debated among modern Sunni scholars because it's not clear what actually makes meat halal.
For example the origins of halal slaughter practice in the Prophet Muhammad's time was to ensure healthy slaughter practice and that the meat wasn't killed in the name of an idol, but no meat is sacrificed to idols today.
The Qur'an explicitly says food prepared by Jews and Christians is lawful for muslims:
This day are (all) good things made lawful for you. The food of those who have received the Scripture is lawful for you, and your food is lawful for them. And so are the virtuous women of the believers and the virtuous women of those who received the Scripture before you (lawful for you) when ye give them their marriage portions and live with them in honour, not in fornication, nor taking them as secret concubines. Whoso denieth the faith, his work is vain and he will be among the losers in the Hereafter.
- Qur'an 5:5
There is also a famous hadith even in Sunni books that it suffices to say bismillah over food — thereby making it halal:
Narrated By 'Aisha: A group of people said to the Prophet , "Some people bring us meat and we do not know whether they have mentioned Allah's Name or not on slaughtering the animal." He said, "Mention Allah's Name on it and eat." Those people had embraced Islam recently.
- Sahih Bukhari Volume 007, Book 067, Hadith Number 415