you answered it yourself. of course Turkey can;t check 100% who enters it soil and leaves it. is there a single country in the world that has its borders 100% under control? i doubt it. those IS scum come from EU countries as well, yet they can leave their countries while they were supposed to be on a blacklist by their govts. sorry, but if other countries can't even control their own citizens, then you cannot expect Turkey to run a thorough background check on every single passenger coming into Turkey. Turkey could have done more, and probably will step up its efforts on Turkish soil now that the hostages have been rescued, but to use such circumstances beyond control as 'proof' for Turkey;s so called support for IS is just bs imo. for the same matter pkk members used to cross into Iran, Iraq and Syria freely after attacking in Turkey in the past and still cross, can we then draw the same illogical conclusion about these countries and their support for pkk through 'opening' their borders? Even if Turkey took more action, IS rats would have been able to cross borders just like the pkk and local smugglers have been doing for decades.
if it pleases you, i am in favor of a hard measures, like the so called Roboski incident where 34 smugglers were killed. no risks, just bomb them shady looking smugglers (who also often happen to have weapons and shoot at Turkish soldiers when confronted) and terrorists, still people cried back then about this measure. damned if you do, damned if you dont.
Turkey's initial support for FSA was wrong, i will give you that, but i believe Turkey has nothing to do with IS. especially when the accusations come from the notorious biased PissTV and hypocrite US which is known for things such as lying about Iraq's wmd and OBL's mysterious and suspicious disappearing, so much for their news credibility about the ME. you only want to see things from your emotional point of view.