This is just the way DE programmed their game. In another thread I think I suggested you contact pinballcode.net to see if they'd be willing to make some of these changes you're asking for. As to why Dataeast did it - that's just the way they did - most of the time people do NOT have games set that way, I'd guess that the coding goes "replay surpassed, award a credit... so, show the credits. Even if it's set to award extra balls".
When you reach the credits max, it's just going to stay at 30.