Volume difference between programmes and commercials

In the UK, if you are watching a channel that broadcasts commercials, you will not only be disappointed with the interruptions but you may also be frustrated with the increase in volume for commercials. So you may either use up the battery on your remote control to either completely switch the sound off or reduce the volume. You have to do the reverse once the programme is back on. Why do the television broadcasters do this? Just stick to the same volume for programmes and commercials.