This is an interesting way of doing it. Normally artists issue statements to say they are playing Glastonbury. This year is seems artists are issuing statements saying they're not. Presumably every artist not playing will now issue a statement confirming so, so we can assume anyone who doesn't issue a statement is playing.
Actually, it's just Oasis who have issued such a statement, and to be fair, that's in response to media speculation Noel Gallagher would be playing a solo set at Glasto this year. Though to be fair to the media, that speculation stemmed from Gallagher saying he wanted to, and making a plea to Michael Eavis to book him during an interview. Though to be fair to Oasis, they only really felt the need to issue a statement because some media types were speculating that Gallagher's bid to do a solo set at Glastonbury was part of some attempt to launch a solo career. To be fair to those media types, Gallagher has been doing all those solo dates of late to promote Oasis hits album 'Stop The Clocks', and he did recently tell NME that a solo album is on the cards, so you could see why they might draw such conclusions. Though to be fair to Oasis, Gallagher was accompanied by bandmate Gem Archer on those aforementioned solo dates, and one solo release doesn't necessarily mean a solo career or an end to Oasis.
Anyway, I'm bored of being fair to all these people, so let's just run the Oasis statement posted on their website last week: "Oasisinet wants to qualify a few stories running across various music and entertainment websites over the past few days that have been a little wide of the mark. Noel Gallagher is not preparing to launch into a solo career nor will he be playing at Glastonbury this June, as have been strongly suggested in some circles. He is an avid fan of the festival and will no doubt be on site checking out a few of his favourite bands again this year. Noel, along with Gem, have had a great time performing their semi acoustic shows recently and will continue to do so when it feels like a good idea, however Glastonbury is not on the agenda".