The thing that you are disregarding is that pool rules can be
different
depending on where you live/the type of pool you play.
In US 8-ball rules I think that officially you are allowed to pot an opponents ball
as long
as you hit your own first (As Martin said above). After doing this the turn passes to the opponent and he/she has to play from wherever you left the white ball.
If you are playing the UK rules then a shot like this would be deemed a foul and you would be given two shots.
The same differences occur with the issue of where you have to play a shot after a foul. In US 8-ball you get the ball in hand (as you do on this site), in UK 8-ball you will play the ball behind the line/on the line/in the D.
Just because the rules on here are not the same as you are used to in real life does not mean that people are "cheating" or that someone does "not understand the rules" ;)
What you're both highlighting is the need for a UK 8-ball room. I would rather have one and would play in it over a US rules games but having a go at people who are complying fairly with the rules of the site (and who may not have even played UK rules before) is slightly unfair IMO.