"Is that cloning or theft? Should clones be welcomed or abhorred?"
Easy. Clones should be welcomed.
1) They put innovation pressure on the original, benefiting everybody.
2) They put price pressure on the original, benefiting everybody.
3) They may create a better platform, a better product than the original, benefiting everybody.
Everybody wins. Except when you look at the motivation to create original products in the first place. Will the clones lower the reward and make it less beneficial to be original?
Hardly.
1) A truly original and inovative product will take some time to clone -- there will be a lead, in which user base/fan base/multiplayer communities should create critical mass.
2) Playing it right, the original *will* have goodwill. In other words, all things being fairly equal, people will likely stay with the original.
3) Originality is a scale, not a binary concept. Games are more or less original. Per (2) above, clones will need to compete in originality just like their inspiration did. When each clone out of many tries to be a little more original than the next, they may arrive at a quite original game, per Darwin. This could happen even though they started off at a lower plateau of originality than the concept originator. Think StarCraft.
4) In this sense, everyone is (or must be) original to be relevant. Originality is not at risk.
I hope that didn't sound too confusing :-)