Why do I see +1.5V on Din of a WS2811-based LED strip?
The 1.5V seen on DIN is probably just the level the input floats at with nothing connected.

This can be checked by placing a large value resistor, say 10K etc, between the input and ground. The resistor should pull the input low and the pin should read 0V.

On some occasions it may happen that a circuit's pin is being driven at a certain voltage. If you connected it to an output on an Arduino that was at Vcc or GND, it will short and may damage the Arduino due to the excess current. This can be seen in the design of Arduino boards with FTDI USB to Serial chips. The FTDI TX pin is driven high when USB is connected, however, if a user set the Arduino RX pin to output low, there would be damaging current flow. So they put a 1K resistor in between the two pins to limit the current if this happens. 

The same thing can be done in the above circuit where it is not known whether the pin is being driven or not. The resistor will limit the current if it is, preventing damage.