Three-point seatbelts are recommended by numerous safety authorities to protect passengers, but there are no laws mandating them on Part 125 aircraft for passengers. Barriers to having this type of seat belt include:
1) Cost/Benefit analysis -- serious aviation accidents are rare, and it can be hard to determine if a 3-point seat belt will make a significant difference (the same applies to seat-belt air bags)
2) Passenger perception of comfort -- particularly on long-haul flights and those who are trying to sleep may find the design restrictive
3) Need to redesign seat to incorperate the seat-belt mechanism, as well as needing to increase the strength of the seat to absorb the loads the seat will now need to take
4) 3-point seat belts may not allow for adjustment for a wide range of heights...certain children, for instance, may require a booster seat to use the 3-point seat belt when a lap belt would have been fine for them.
The UK AAIB after the Manchester Air Disaster recommended the investigation of rear-facing seats and 3-point seat-belts as they are believed to be more protective. But the only aircraft you will see it for passengers is corporate aircraft
...which is an interesting commentary if you ask me.
The sad fact is that a lap-belt is a poor compromise for safety, even when combined with the brace position (far too few passengers know how to adopt the proper brace position in a split-second anyway).
[Edited 2006-09-09 15:29:33]