Yesterday, on November 21, this game went to v4.0, and joined the free SEGA Forever Classics. With this, came ads, which can be removed with an in-app purchase for $1.99

But, if you got S22013 before 11/21/2017, when it cost $3.99 to buy it, you can "restore" the in-app purchase. It will ask you to enter your password in, and in 5 seconds the ads will be no more.

So, if you got the game before November 21st, you can do this. If not, I'm afraid you will have to do the in-app purchase.