The algorithm for choosing the tips to reference "prefers" tips with larger cumulative weight. Those precomputed legit transactions will have much smaller cumulative weight (than other tips), and so they will probably not be referenced by others.
Isn't there are problem with merger of a split tangle then. If the network was split, then tips of the stronger part will have greater cumulative weight and will always be chosen to be referenced. The weaker subtangle just dies off.
The party which is interested in the survival of the weaker subtangle would "connect" it to the stronger subtangle by referencing 1 tx from here and 1 from there. If both subtangles are legit, they'll quickly merge.