Consider this:
- On major roads in NYC, the speed limit tends to be 25mph
- Your bike goes 15mph
- Fastest NYC walker tends to do 5mph
So, you're going right in-between the two speeds, but you're going at 3x the speed of pedestrians but more than half the speed of the cars.
Do you think it's OK to drive a small vehicle or ride a motorcycle on the sidewalk? Or, how about walking down the center lane of a major road?
It's a safety issue - it's not OK. A pedestrian may appear any time. Meanwhile, if there is traffic, you can weave and lanesplit a bit.