Banksy has done more in this painting than just “observe” and “write down” (i.e. paint) what he sees in the UK parliament. Banksy is clearly making a political statement and the timing of it (during Brexit discussions) along with his body of word indicates that he has loftier goals than literally painting what he sees.
Another artist might have represented the parliament in a more realistic way and then the painting would be closer to simpler observation. However, even a realistic painting is more complex than this as the artist is making numerous decisions including, for example, what to include / omit in the painting.