Very good question sister..
and i am sure many of us muslims might have this question in their mind and would like to know the answer..
1. In Moharram it was Yezid who stopped water and food for the Ahle bayt. It was the work of Yezdis not to give water and food to the muslims...Hence by distributing Food and Water in Moharram we are actually doing things which Yezid dint do and hence we are doing right.
We give food water juices Sharbat in the name of Hussain signifying that Imam Hussain ( (Rahmatullahi Allaih)) dint had any shortage of water...wo Dono jahan k maalik ke Nawaase the aur unke liye paani ka ek samandar bhi wahan haazir hojaata but it was Allahs will that Imam Hussain's martydoom become a lesson for all Muslims coming after him and hence Imam Hussain and Martyrys of Karbala laid down their lives to teach the world lesson of Sabr and Imaan.
2. It is proved by Hadees and Sunnat that we can give our Blessings as a Reward to the people who have passed Away. So doing charity like feeding the poor and giving water to the thirsty these actions bring a lot of swab and we offer these sawabs to the Martrys of Karbala which is very much permissible
Apne Amaal ka swaab aap Shahdaae karbalaa ko dete hain..aur bhuke ko khana khilaana pyaase ko paani pilaana bahot bada swab hai...this can be looked like this also...
Shirk and the veneration of Graves