O.K, I'll reword my initial statement:-
The PL had a range of punishment options available to them.
There was no precedent to draw from regarding this offence (nobody had previously committed it), hence a fine was not an unreasonable punishment.
People keep talking about West Ham playing an illegal player. This is not true. Tevez was, at all times, legally registered with the PL (confirmed by the PL in their earlier statement).
Another misconception with some people is that West Ham were guilty of playing a player who was owned by a 3rd party. This of course is nonsense, as it is totally legal for players to be owned by a 3rd party.
The problem with the original Tevez contract was that it contained a clause that allowed the said 3rd party to sell the player without West Ham's permission. How anyone can construe that as an advantage gained by West Ham is beyond me.
The PL have already stated that if the original clause had been disclosed at the time of the signing, the registration would still have gone ahead, but they would then have insisted that it be removed (which was what happened later).
The rule that West Ham broke was originally designed to stop anyone owning more than one football club, hence having a potentially damaging 3rd party influence, so it had never been applied to potential 3rd party influence on a player before. So, sources close to the club say that they weren't even aware that they had broken any rule. The PL accepted that the wording of the rule was confusing (they are going to reword it before the new season starts), and hence of all of the punishment options available to them, they chose a fine.
Common sense ruled (in my opinion).
Sorry about the long rant, but I have read just about every word available on this over the last few months, and thought I'd share the info.