I'll allow that I don't know very modern convention. I have a grasp of what it means to be polite and how to behave in a considerate manner, but there are also rapidly updated trends into which I have not tuned. But nor will I say that I should endeavor to be made abreast of them.
A few years ago, I was enjoying karaoke with some friends at a bowling alley/bar in Nordeast. Some of these friends were people I'd seen frequently, a couple were such that I hadn't talked to in years, and a few more were people I hadn't known but liked immediately. And I like karaoke, and I like it better when we've had a few drinks, and we were drinking that night.
My turn came up and I took the microphone. I had signed up to perform "Our House" by Madness, nice and simple, a catchy favorite. I hadn't finished the first verse when suddenly this drunk, screechy blonde woman came reeling over, announced that this was her favorite song and immediately began "helping" me sing. She was short, about 5' 3" or less, of slight build, with a cute face and long golden hair. I was given to understand that it was her birthday and a couple tables had been pushed together to seat her large group of friends. Her "assistance" to me consisted of slurring intermittant lyrics, waving hysterically, and shrieking to her friends that it was her birthday.
I was a little put out that I wasn't permitted to perform as well as I might otherwise, but to show there were no hard feelings I thought I'd buy the girl a drink. It was her birthday, after all, and she was quite merry. I went to the bar and ordered the tastiest drink I knew, an amaretto sour. I tasted it myself to be sure of its purity, and it was delicious.
I walked over to the girl's table and flagged her down; it took her a moment or two to refocus and recognize me. I said, "Happy birthday," and presented her with the drink. Her face fell and she stared at me very hard. A very tall man who resembled a young Justin Timberlake in white and blue basketball clothing loomed behind her and slid one protective arm around her chest, and he also stared very hard at me. I looked back at each of them and assured them it was just a drink for her "help" at the microphone and to wish her a happy birthday.
Her expression darkened and she took a sip of the drink. Immediately she screwed up her eyes, stuck out her tongue, and made hacking, gagging noises. "This tastes terrible!" she informed me. "What is this?" I informed her it was an amaretto sour and a quite good one. She showered me with a few more unfavorable adjectives, and her attendant idiot man-child glowered at me.
I regret that I lacked the wherewithal to snatch the drink back and return to my seat; as it happened I only glanced at each of them sullenly and returned to my seat. I'm now given to understand it is impolite, and perhaps even threatening, to buy a drink for someone you don't know. I don't feel this indicates society is evolving in a direction to my liking.