Each time I try to send credits to someone I am told "Error! You cannot send these credits as they are part of a promotion. This limitation is important to ensure clean play."
I see where they are coming from but it's stopping me from sending credits to everyone.

I'm pretty sure it's trying to stop you from manipulating the reward system for getting a friend to join the game. Especially if you're trying to send those credits to that person.
I'm pretty sure it's the way the No Milking rule is being enforced.
There's no bug here.

If that was the case then why does it stop me from sending credits to everyone? Not just that one person?

Because the credits that you recieved are only for 100% personal use. The only way you can use them is through legal transactions on the voodoo market or witch hut. This is again, "to ensure clean play."

I got the same thing and I got the credits from selling voodoo cards

But what if someone sent me one as a favor and later on the friend needs one?

Here is how it works:
-You cannot send promotion-earned credits (ie referral credits).
-Credits earned through market are NOT promotion-earned credits. They are as good as purchased credits.
-Basically, there is a counter that counts the number of the credits you have earned through promotion. This counter can only rise.

To send credits, you must have more credits than total credits earned through promotion (more than the counter). Then, you can only send the extra credits.

For example:
-Player A has 100 credits.
-He has earned 40 through promotion.
-He can send 60.
-If his credits drop to 35, he will not be able to send anything.
-He will need to get above 40 again to be able to send.
-If he gains 5 more credits through promotion, then the limit will rise to 45.

but i never gained any promotion

What do you mean by promotion?

Maybe the referal system? If someone sign up using your referal link like this
http://s2.piratesglory.com/promo.php?id=xxxxx (the x's representing player ID) you get 1 credits
And if they reached certain fame points you get credits also, and if they buy credits you earn 10% (I.e. If they buy 10 credits you get 1)