"There is baseball practice tomorrow."
There is also the fact that words with only kanji and の tend to have the の disappear over time. The correct spelling will become 野球練習 in 50 years, just like how 漢字辞書 doesn't have a の unlike the newer 英語の辞書 which does. Even now, a Google search shows both results with and without the の.
shouldn't 明日に at the beginning also be accepted? In other words, can't the time just be the time without being the topic?
Yes, the time doesn't have to be the topic, but you would not use the particle に. 明日 is an adverb, so you would just say 明日、野球の練習があります.
For more about time expressions with and without a particle, see Maggie-sensei: http://maggiesensei.com/2016/01/28/time-expressions-with-and-without-a-particle/
Baseball practice is tomorrow.
It sounds like you didn't know when baseball practice was, and someone is telling you that it's tomorrow.
There is baseball practice tomorrow.
It sounds like someone is explaining tomorrow's schedule.
There might be cases where the meanings overlap, and I think it's reasonable to try submitting an error report saying "my answer should be accepted", but the contributors might consider the sentences semantically different.