Wa alaykum Salam Wa Rahmatullahi wa barakatuh,
Imagine a person who has a job interview, they are careful in how they look, sit and interact with their potential bosses. When people are with those they really respect and want to please, they do all they can to have a certain "adaab". They try not to make any bad jokes, to be patient, to sit straight and not slouch, to not offend the other person. - To make a great first impression. Nobody who seriously wants to get the job are going to put their feet on the table.
As Muslims we have a lot of pride in Islam and more respect for Allah and all things related to the Deen (including the holy Quran, the Prophets, even the other holy books such as the bible and the torah ) than we have for our own parents.
In a lot of Islamic cultures, people have immense respect for all books. Children are raised not to throw any books, or to not play / disrespect food etc Though there is no direct evidence instructing us to not throw books etc, but, in general its a good idea to have a certain level of respect for such things, and there is evidence from Quran and Sunnah for that. The general ayaat in the Quran that mention the blessings we have and to ponder those, and the instructions for being humble, patient etc There are also specific instructions in the Quran and Sunnah, such as the hadith where Muslims are instructed not to relieve themselves in the direction of the qiblah. That we use our right hand for good things such as eating, or giving and receiving items to other people, and left for cleaning etc. Respecting elders, teachers, the food we have etc.
So altogether, this is what is known as Adaab.
Islamic Manners. Depending on the culture, some people are raised at different levels than others. It can also be kind of daunting or strange for those new to Islam. i.e. Its seen as being disrespectful to sit with your legs stretched out and feet pointing at others... particularly if they are elder and respected members of society. And of course, people also do not like to sit pointing their feet at the Qiblah, or the Quran.
It's also a posture that is seen as not befitting a "respectable person" like a student of knowledge etc.
We may sit inadvertently like so, and there is no sin on us. But, if the thought comes across our mind, "hey, i'm sitting and my feet are pointing at the Quran".. then why shouldn't we change and sit in another way? This is where Shaitan puts pride in a person's heart and with excuses such as "well, its not "wrong", so why not?" - Some scholars have said this is wrong and a sin, others class it as disliked but not a sin. The issue here is about respect. If we are to place the Quran with books, the Quran is always going to be on the top, never placed below anything, even reading glasses.
Like what is mentioned before, there some things which are just good manners, and not necessarily instructed by Islam. We should learn what is from Islam and abide by those, and other things which are not contrary to our Deen and are reasonable, we should do our best to understand and abide by, especially if it is offending to other people i.e. the elderly of our communities.
I remember one incident in the UK a few years ago. The brothers in a masjid were adamant to pray in the mosque with their shoes. - The masjid Imam and elders had told the brothers to remove their shoes before entering the prayer area. - The brothers proceeded to bring evidence where the Prophet (s) and his companions had prayed with their shoes on the carpet.
Disregarding the rules for this particular mosque, some brothers entered the mosque with their shoes and prayed... infuriating the masjid elders. This is a real example of disregard for Islamic adaab and a total lack of understanding of Islamic principles. This is extremism, which in Arabic is called "Ghuloo".
People act like this, and then they wonder why there is friction between young and old, etc
There is time, place and wisdom in carrying out the Sunnah.
As long as people dont go into any extremes.
Finally I asked him “What’s the daleel on all this?” and he got very upset.
I think it be wise to apologise to him, inshaAllah. Asking for daleel is right for things that are construed as being from Islam. If someone said you are "sinning" by pointing your feet at the Quran, or towards the Qibla, then by all means object and ask for Daleel. But, otherwise, its good to just make the slight change to keep things peaceful, afterall, we hold our Islam highly in our hearts. We know Allah is watching and out of our love for Him, we will do our best to clear our hearts and minds and that of other people around us, of any such thoughts.
Wasalamu alaykum.