You’ve got the basic baseball metaphor right – the speaker is implicitly comparing the idol to a batter who swung at the wrong kind of pitch. I’ll break down a few of the key phrases below.
フリーバッティング (“free batting”)
means batting practice, and according to 大辞林 it refers specifically to a type of practice in which the batter is allowed to pick what kind of pitch he wants. I initially took this as the speaker's own metaphor for the idol's philandering, but on reflection I think you're correct in interpreting it as the name of the love hotel where he was caught. Presumably, it's this name that then inspires the speaker to come up with the baseball metaphor that follows.
refers to batting selectively or “waiting for the right pitch" – that is, planning one's at-bat and taking a careful, disciplined approach, rather than simply chasing any pitch that comes along or trying to decide after a pitch is thrown whether it is going to be hittable or not. The "right pitch" can vary depending on the state of play, the pitcher's strengths and weaknesses and pitching patterns, and the batter's objective (e.g., getting on base vs. hitting a sacrifice fly or bunt). It can also change during the course of an at-bat.
is a hypothetical example of a 狙い球 – a low, inside "straight" pitch. (That is, a non-breaking pitch, like a four-seam fastball, as opposed to a curveball or a slider.)
So we can translate this part of the passage as
A love hotel called "Free Batting"? Give me a break! If you’re going to fool around, don’t just swing at anything – wait for a non-breaking pitch that’s low and inside.
Without more context, I don't think it's possible to say exactly what is meant by this metaphor, or whether there’s any real significance to the specific pitch that’s named. But the basic idea seems to be that the guy should have been more selective in choosing a partner and/or a venue for his philandering. That is, either the woman herself was somehow unsuitable, or he should have taken her somewhere other than a love hotel. And possibly these two ideas are linked: He shouldn’t be fooling around with the kind of woman who is content to be taken to a love hotel.