Texting has largely replaced phonecalls. Therefore most people expect you to get back to them within a couple of minutes, not hours.
I have used, and have seen others use, texting in real-time applications where immediate response is required. Like texting someone that you're at the door (and the bell doesn't work) instead of calling them.
Generally the younger you go, the more texting is expected to be instant communication.
While I don't disagree, I think such people are wrong. Even if you're at my door and the doorbell is broken, if you don't call, I might miss your text. I won't apologize because you didn't call. To me texting is asynchronous and if you want real-time, synchronous responses, you should call. The expectation of any kind of timely response is ridiculous, IMO, and is completely inappropriate. I could be doing any number of things that require full concentration (work and sleep come to mind), be out of cell range, have forgotten my phone, have a dead battery etc. I have seen people get upset over not having their texts responded to and I judge them, rightfully and harshly, as being inconsiderate, demanding, and selfish. If it is indeed a cultural norm, it is an extremely inconsiderate norm indeed.
Don't get me wrong, you're welcome to feel however you like. But many don't really see any difference anymore between texting and calling, since both go to the same device. Your objection "you should call because I may be out of cell range or have forgotten my phone" strikes me as illogical especially for the kind of synchronous situation like "I'm at the door, your doorbell doesn't seem to work."
The difference is that texts are easy to miss. Not only that, but by calling you have established synchronous communication. If the recipient doesn't pick up, then you know you have no expectation of a response while the opposite is true if he does. On the other hand, there's no guarantee that your text message was even delivered, this assuming that your recipient should receive it and act upon it is quite presumptuous. That's the difference. Calling establishes a communication channel that texting does not. If one does not pick up the caller knees there is no such channel established and thus no expectation of communication.
I have used, and have seen others use, texting in real-time applications where immediate response is required. Like texting someone that you're at the door (and the bell doesn't work) instead of calling them.
Generally the younger you go, the more texting is expected to be instant communication.