At least running VICE 2.4:
PRINT PEEK(53280)
254
POKE 53280,14
PRINT PEEK(53280)
254
It's clear that only bits #0-#3 are used, and that the "correct" value is obtained with AND 15, but why does the 6510 set the upper bits to 1?
The 6510 CPU doesn't set the unused higher bits to 1; rather, there is no memory backing the higher bits in the appropriate VIC registers. So what happens is that the CPU puts whatever 8-bit value on the data bus when writing, but the VIC chip only stores the lower 4 bits; then later, when the CPU tries to read from that address, it puts 53280 on the address bus, and the VIC chip needs to put all 8 bits on the data bus. But it has only stored the lower 4 bits; the higher 4 bits will need to be faked up as 1.
Of course, it could also pick other fake values for these extra bits (e.g. it could use all 0s); the point is, it has to be a fixed value, since the real 4 bits were never stored anywhere.