All omniframes owned by the player have their shards available for purchase in PPC except for Nier.
Without the ability to by shards, spending **resources **(cards/tickets) on character you cannot continue to grow (learn new skills via rank, etc) effectively results in a wasteful use of account-limited resources.
The nier constructs will eventually be outpaced by the other (trainable) constructs you own and also by newer constructs in the meta; this discourages spending on the collab period and encourages skipping out instead.
||Lastly, I don't think that having purchaseable shards for previous owners will hurt collab sales, as explained below.||