This issue was brought up by Tom Petty's publishing company, which contacted Smith's publisher about giving credit on the song. It is unknown as to what legal proceedings came out of this, if any, but it seems like Smith's people awarded Petty and Lynne songwriting credit without much resistance, seeing as the song only came out last April. A radio show actually compared the two songs by playing them on top of one another and changing their speeds and musical keys to match, and, sure enough, the chorus melody is right on. You see, in the world of copyright, you can only copyright two things while writing a musical work: melody and lyrics, nothing else. The lyrics aren't similar, but the spacing between those few notes in the chorus melody is the same. Whether Smith and his co-writers were struck with cryptomnesia or just plain didn't know about the Tom petty song, there's really no shame in accidental plagiarism. There's a whole lot of songs out there and some lines are bound to get crossed, so there's nothing wrong with that happening as long as its not intentional and you give credit where credit is due in a timely manner after being informed. It seems like both sides in this particular situation handled it pretty well.
Petty has also known to be pretty cool about people ripping him off accidentally or otherwise. He usually just brushes it off when it happens. So, here's some other songs that were claimed to be similar to some of Petty's other works:
"Dani California" by Red Hot Chili Peppers and "Mary Jane's Last Dance"
-Chords and lyrical themes were deemed similar. However, chords and themes are non-copyrightable. Petty let this one go.
"Last Nite" by The Strokes and "American Girl"
-Chords and harmonic sequence in the beginning of the song. Again, you can't copyright chords or other harmonic stuff going on in the arrangement. However, they do sound pretty similar and I have heard of exceptions being made for plagiarizing defining characteristics of a song. Even though The Strokes later admitted to copying "American Girl," but Petty laughed it off and even had The Strokes open for him on one of his tours.
However, those were both pretty ambiguous territory and didn't technically cross the line into copyrightable material (remember, melody and lyrics). I guess Petty's finally had enough.
So, why does this keep happening? Well, Petty writes simple, catchy melodies over simple, catchy chord changes. It's hard not to step on some toes. That kind of explanation won't hold up in court during a copyright lawsuit, but it does give you an idea of why it happens.
If you can think of any songs that sound a little too similar to others, share them in the comments!