Is there a reason L does not arrest Light immediately in the episode “Wager?”

In this episode, L has caught Misa and informs Light that he has captured her. He then just leaves Light free. Why? L is already very sure that Light is Kira. He is basically 100% sure that Misa is the second Kira. He knows that Light and Misa are connected, he knows the second Kira is infatuated with the original Kira. Surely for safety reasons he just takes Light into custody as well?

Now, I know L doesn’t have the same information as the viewer does (a common mistake viewers make when discussing smart characters). However, I don’t believe that locking up Light who he already suspects is Kira after catching Misa is a massive leap in logic. In fact I would argue it’s common sense. He allowed Light time to come up with a contingency plan. If he had arrested Light on the campus, Light would have been caught completely off guard. Which L knew as he surprised him there and had stolen Misa’s phones without Light realising. This would have led to Kira being unable to kill any new criminals that had been broadcasted, and would have proved that Light is Kira. Of course, L wouldn’t be 100% sure of that, but if Light WAS Kira, it would be certainly worth the chance. If Kira continues killing new criminals, then L could just release Light and say, “well your gf is the second Kira so we took precautions,” and it would be fine.

I guess the reason is plot, but considering L is the world’s greatest detective, this should have been an easy plan to do. There was no reason to give Light time to come up with contingency plans, and no reason to wait for Light to willingly come forward, as by that point L should have realised that if Light WAS Kira, he would have a contingency plan.