might have to do with catching cheaters? On most sites the ptc banners are supposed to be viewed one at a time (yes that does take a very long time, might be why I dont get to the ptc pages on sites a lot) but I guess the thinking on this is, people are paying for the ads, if we view them one at a time we will more then likely ``read`` them, if you have 3 or 4 open at once you may not be reading them , but of cource the down part is, if you already belong to the site, OR its a ad that 3 people sent and then its either click it and wait for your money/points or just skip it and miss out on a 1/4 penny or 10 points (just a example LOL)
of cource I could be wrong so hopefully a w/m or 2 come and answer your question, it is just a guess for me