I think it depends heavily on what state your in, at least in the US.
While it's a bit differant, understand that related issues have been a big concern in the US for a long time. They most famous case of this kind of thing has to do with the Kitty Genovese case. In short she was stabbed to death while screaming for help while a lot of bystanders just ignored it and let it happen.
One of the big concerns in the US however is not so much apathy, as the wikipedia article reports, but a fear of of being punished for trying to help. The classic case of a victim sueing a paramedic, or someone intervening to protect someone else from an assault being themselves charged and convicted of assault and so on. This has lead to a lot of so called "Good Samaritan Laws" intended to protect people who choose to get involved base on intent, however they tend to be very touchy as they decay over time due to victims or villains sueing the rescuers and demanding exceptions, only to be re-affirmed when a major "Kitty" type incident takes place, and then begin a process of erosion again.
In the case of the guy "dying in quicksand" it's a little less straightforward, but the bottom line is that if you decide to get involved, there is always the chance that the guy is going to sue you. Let's say you throw the guy a rope, pull him out, but there are complications and he gets hurt while your tugging, at first he's thankful, but then six weeks later your facing his lawyer due to your incompetance in recueing him. I'm sure we've heard of stupid court cases right up there with that (Thief sues home owner he's robbing for unsafe stairway or so on). Likewise, just by calling someone you are taking a degree of responsibility, if the guy dies while waiting for the authorities, or he is injured, questions like "well, why couldn't you have called 5 seconds earlier" or whatever can come up. This is one of the big reasons why there are issues with Anonymous tips on things like 9/11. The people making the reports don't want to be on record and any way held accountable, and of course emergency services want people for the records. This in of itself has caused problems since many emergency services put the anonymous calls to the bottom of the list since they could be crank calls.
Of course the flip side to all of this ridiculousness is people's general tendency to abuse what freedom they happen to be given when it involves other people. The reason why such good samaritan laws tend to gradually erode after every big issue is because you wind up with jackoffs who decide to abuse them to take advantage/hurt/screw with people because they can, or just do stupid and careless things. You know, the guy who decides it would be fun to "accidently" throw a live electrical cable into the quick sand to pull the guy out and then say "ooops" later, or the guy who decides to sit there for 20 minutes and then call the authorities knowing it will probably be too late because he's a sadistic twit, or the guy who decides to intervene in an assault just so he can have the pleasure of getting a free hand with excessive force charges. Excessive force in the case of stopping an assault being a common one, and also what concerns people, and can vary with continuum of force state by state. For example if you see some guy raping a girl at knife point, you whip out a .45 and blow the dude's brains all over the wall, and it raises a lot of questions since the guy with the gun was not directly being threatened, and could have potentially run away to get help, or numerous other things. Not to mention the whole "side" issue of the victim claiming psycological trauma from not only being nearly raped, but also being splattered with brain chunks. In some states that might be fine, in others... well not so much. In general in self defense you can go one step higher than the person attacking you to protect yourself, the steps determined by the specific state policies and it can get complicated. Contrary to belief you don't have to use "equal force" in most places assuming you can't flee. However again, the whole issue is that in a good samaritan-type intervention the guy doing the intervening is not in any danger usually before they get involved and that can become a key point of law, especially in the more left wing states that really frown on personal empowerment/armament/etc...
I'm just explaning things as I understand/learned them, it's been a very long time since I took Criminal Justice and related classes though.
The overall point is that like many things in the USA at least, a lot of it depends on where you are. Concerns over liability are also a big part of this. In general one of the biggest reasons why people don't get involved is the fear that it will backfire on them. It's sad, but a lot of people aren't going to save another person if it means going to jail, getting sued, or other things, and that can be a very real fear. It's simply safer to not get involved, and honestly it can be argued that there is a differance between not being a hero, and being a totally detached bastard. It depends largely on the circumstances.