they all pay out in uBTC. I have over 50 small transactions,all in uBTC. Now,suppose I want to spend this amount,will the transaction fees eat away most of it?
Yes.
And they know this. They take advantage of uninformed newbies convincing them that their efforts will result in spendable value while hiding from them the fact that a huge portion of that value (many times all of it) will have to be used to pay transaction fees due to the overwhelming number of inputs required to use it.
Meanwhile, newbies see an ever increasing balance and recruit other newbies offering suggestions on how to earn "free bitcoins" with these sites.