Because, strictly speaking, eating or drinking from gold or silver vessels could put one on slightly dodgy territory when it comes to religious observance. Ibn Battutah is almost certainly referring here to a family of hadith that expressly condemn eating and drinking (and ritual ablutions, etc.) from vessels made out of precious metals. For example, Sahih Muslim 2067g:
'Abd al-Rahmin b. Abu Laili reported that Hudhaifa asked for water and a Magian gave him water in a silver vessel, whereupon he said:
I heard Allah's Messenger (ﷺ) as saying: Do not wear silk or brocade and do not drink from vessels of gold and silver, and do not eat in the dishes made of them (i. e. gold and silver), for these are for them (the non-believers) in this world.
It has been suggested that this attitude was a direct reaction to the high status of silver and gold vessels in Sasanian Iran, and that it developed only as the Islamic conquests progressed into Sasanian territory. It is not surprising, then, that the putative restriction on the use of such vessels seems to have been flouted most in what we might call the "Persianate world" (the area in which Persian linguistic and cultural influence surpassed that of Arabic), which included Anatolia as well as Iran proper. Though here, too, there was certainly religious pushback. The twelfth-century "mirror for princes" Bahr al-favāʾid has a relatively lengthy section on the subject (quoted in Melikian-Chirvani):
Vessels for liquid substances. Bowls containing food and beverages. If made of gold or silver, its use is forbidden to men and women. It is improper to use it for eating or drinking, for the Prophet, may God's grace be upon him, has said: "Verily, he who drinks out of gold or silver vessels is like a man in whose stomach the fire of hell would be burning...'.
...Consorting with someone who uses gold and silver vessels is forbidden because gold and silver were created to define the price of wares and the going rates of merchandise. When vessels are made from them, locked up in a prison, and deposited in a treasury, they can no longer perform their role in arbitrating [qāżīʾī], and maintaining public order, [muqavvimī], life becomes difficult, and God's precepts are broken.
...To use it (gold and silver plate) is to imitate tyrants and oppressors [dar istiʿmāl-i ān tashabbuh ast bi jabbārān va gardankishān].
...It drives the poor to despair. When they consider the luxury of the rich, they get desperate at the thought that these have gold and silver plate when they cannot even get earthware. It has, therefore, been banned, so that the poor should not grow desperate.
As the example of the Sultan of Birgi might suggest, however, the sumptuary laws laid down by these hadith were not always observed by Muslim elites. (In fact, according to Melikian-Chirvani, they were usually not observed by "even the most pious characters among the ruling classes.")
For example, the same prohibition on gold and silver vessels was often extended to include cookware—a shame, as silver conducts heat really well, but this was one reason why medieval and early modern cooks preferred
cooking-pots ... made of stone, or as a second-best those of earthenware: only as a last resort should [a cook] use pots of tinned copper. There is nothing more abominable than a food cooked in a copper pot which has lost its tinning.
(The quote is from a twelfth-century cookbook by Muḥammad b. al-Hasan al-Baghdadi, translated by A. J. Arberry in the '30s, and reprinted in the 2001 volume Medieval Arab Cookery; it is repeated, with minor variations in Muhammed b. Mahmud-i Şirvani's creative fifteenth-century Turkish translation. See also my lower-level comment.) But does this mean that in practice silver cookware was something entirely unknown in the Islamic world? Of course not! Thus, the sixteenth-century chronicler Hasan Beg Rumlu tells us of the powerful Safavid statesman Najm-i Sani that "Daily up to one hundred sheep were used in his kitchen, and food was cooked in thirteen pots of pure silver."
Bibliography
They're a bit dated, but two of the essays in Pots & Pans: A Colloquium on Precious Metals and Ceramics in the Muslim, Chinese, and Greco-Roman Worlds (Oxford, 1985) are probably still among the fullest treatment of the issue in academic writing:
Michael Rogers's contribution on "Plate and Its Substitutes in Ottoman Inventories" in the same volume (117-136) is less helpful, but I suppose still worth a look if you have the book open. You might compare it to Colette Establet and Jean-Paul Pasqual, "Cups, Plates, and Kitchenware in Late Seventeenth- and Early Eighteenth-Century Damascus," in Suraiya Faroqhi and Cristoph K. Neumann, eds., The Illuminated Table, the Prosperous House: Food and Shelter in Ottoman Material Culture (Würzburg : Ergon in Kommission, 2003), 185-197. n the Ottoman case, see also Hedda Reindl-Kiel, "Breads for the Followers, Silver Vessels for the Lord: the System of Distribution and Redistribution in the Ottoman Empire (16th-18th centuries)." Osmanlı Araştırmaları 42 (2013): 93-104 (but really only 101-102; academia link).