If you are not going far a taxi is not really any more or less safe than a bus ride, where there is no carseat or booster required.
When you want a taxi, you are going to have to call for one that has seat belts in working order (believe me, they rarely do) or one with a car seat. You will have to wait 30 to 60 minutes for one unless you are lucky. It's a real pain in the bupkus.
The human species did survive through my childhood without car seats and boosters. Each state has different laws. It's all a matter of how much of a risk it is and if you are willing to take it.
My state wanted my 9 year old daughter in a booster because she was not quite 48 inches tall yet, while all 63 of the other kids her age in her grade were over that height.
I am a fan of making your own choices.
Carrying an infant on a plane is also "not safe" but thousands do it every day. Why do the airlines allow it? a) those people will drive instead of buy another plane ticket b) It's still safer to fly unrestrained than it is to ride in a car. We bought a ticket for our daughter on every flight she ever took and put her in a carseat. In florida we rented a car and put in our carseat.
But we never called for a taxi with a carseat. It's 3 miles out of a million she will ride in a car and we just don't see it as a significant risk compared to all others, especially buses with no seat belts!