Yes, it's a function of the way in which asset issuance works over the transaction model, and the fact that we haven't specifically made a provision for follow-on issuance. Here what we envision for a future version:
a) When you create an asset, you create a fixed number of units now, plus you specify a maximum total number of units that can ever be created for it. (If maximum = fixed, no follow on issuance is allowed.)
b) The private key used to sign the asset issuance transaction can also be used to sign transactions that create more units of that asset, up until the original maximum is reached.
Would that be a good design for your purposes?