“he let Amnon and all the king’s sons go with him.” That David did not suspect that something evil was being plotted when Absalom asked specifically that Amnon be allowed to go to his sheepshearing banquet (2 Sam. 13:26) is more of David’s blindness concerning his sons. Why didn’t David remember that Amnon had raped Absalom’s sister (and David’s daughter!) only two years earlier and now Tamar was living in Absalom’s house as a “desolate woman” (that is, unmarried and without children), and thus was a constant reminder and source of bitterness to Absalom? Especially given that in royal societies brothers were always trying to eliminate one another to gain power, especially the throne itself, and especially since Nathan had told David that one from his own house would lie with his wives (2 Sam. 12:11), which was a clear reference to someone trying to take the throne from him, it seems David would be more on the alert that there would be serious trouble from his sons. But David’s blindness when it came to his sons prevented him from seeing the danger and taking measures to prevent it.
“So Absalom made a banquet like a king’s banquet.” Although this sentence is missing from the Masoretic Hebrew text, it was almost certainly in the original and omitted due to a homoioteleuton (words that have the same ending, causing the copyist to skip words). The sentence can be found in the Septuagint, and also in the Old Latin. Also, there is evidence that it was in a Qumran manuscript (Dead Sea Scrolls book of Samuel taken from cave #4). Josephus (Antiquities; 7.8.2) mentions that Absalom threw a banquet and waited for Amnon to be weary from wine (cp. The Anchor Bible: II Samuel, by P. Kyle McCarter, Jr.).
That Absalom would have a banquet like “a king’s banquet,” which meant it had lots of wine, would be natural and would help explain why Absalom would be so confident that Amnon would become “merry with wine.” Also, such a banquet would make sure the other son’s of David would not be in a position to defend Amnon.