I think ball-tampering would be far more common in world-cricket than anyone would like to admit. I remember seeing Kapil Dev picking the seam up against Australia in 1991 and it was so obvious, Ritchie even gave him the "Ohhh, Kapil.......". I've watched Brett Lee do it, I've watched Hansie Cronje do it, I've watched Waqar do it, I've watched Chris Cairns do it etc., etc. on and on it goes.
I've long held the opinion on this forum that ball-tampering is made far too big a deal of and the fact it's made a big deal of lends further proof to the opinion that it's a batsman's game. If England are doing it this series, it's just evened the playing field. They've shown some of the Aussie batsmen (and fans) that their averages flatter them.
Short of using bottle-tops or anything 'artificial', a bowler should be able to do with the ball what he wants. If he can make the ball bend around when bits of seam are falling off it by using fingernails/spit, etc. that's skill, not cheating. It's a calculated risk too because if a bowler is affecting the ball and buggers it up, then it's his fault when the ball stops moving. Plus, having a ball which moves everywhere is useless if you can't bowl it in the right areas so there's still plenty of skill required.
In a lot of Tests, after 20 overs, the ball using traditional means is essentially useless. That means that the batsmen get 60 overs of movement-free batting against fast bowlers (vast generalisation, I know). You want to talk unfair advantages? There's one. There have been many innovations which have helped batsmen over the last 30 years (better willow, designs which make bats lighter but still have plenty of wood, shorter boundaries, etc.).
Name one for fast bowlers.
If anything, fast bowlers have had nothing restrictions placed on them (bouncer rules in Test and ODI cricket, making Mankadding virtually impossible, tightening of LBW rules). You'd have no trouble convincing me that fast bowlers do get an advantage from being able to do more with the ball but in all my years of watching cricket, no-one and I mean no-one has ever been able to show even a little why those advantages are 'unfair'. There's a big distinction between an 'unfair advantage' and an 'advantage'.