You are so simplifying the issue. He got a ripper from McGrath in the first innings at Lords in the midst of one of the great Test spells of all time, was unlucky in the second dig when he played for turn that didn't happen (didn't play it all that well but until that point, every other ball just like it had ripped through to Gilchrist), got an excellent length ball from Kasper which moved away off the deck in the first innings here and a ripping leg-break from around the wicket in the second dig. For a guy who's playing in his first Ashes series, Bell has gotten a disproportionate amount of brilliant bowling so far. In between those ball, except perhaps the second innings at Lords where he and everyone else struggled with Warne, he's actually looked pretty good. When he gets a start or a bit of luck, I think he'll be a good chance for a ton.
That ball pitched just outside leg-stump and forced the stroke. He did have to play at it because he wouldn't have had any idea how far it was going to turn. Imagine if, considering his second-innings dismissal at Lords, he'd left it and been out bowled around his legs. You would have been tearing him a new one (and rightly so).
That's what makes Warne's leg-side line so difficult to play compared to other bowlers. Other spinners you can kick away or play confidently at but Warnie turns it so much, it's very difficult to determine whether it's going to spin back and hit the stumps or whether you can kick it away. Warnie also bowls at a quicker pace than traditional spinners, further complicating the decision. That was a great ball from Warnie and Bell didn't do a much wrong. Anyway, it's all too easy from the couch to berate someone for 'playing at a ball they shouldn't have' in hindsight. Until that point, he'd played the leg-side balls from Warne really well, hitting against the spin into the leg-side. That's why it took a good one to get him.