That's because they see the club finally having money to spend and want it spent on players that they rate.
You may rate Nugent, Bobo Balde, Philip Bardsley, but others may not and will see it as a waste of money.
It's about bringing in the right players, not how much money they spend.
And yes, trust Martin O'Neill etc, but people are allowed opinions on whether they rate a player or not. For example, I think Bardsley would be a waste of money as he's nothing better than what we've got and that Curtis and Nugent are both over rated. If O'Neill wants them, go and get them, but it's not going to stop me being disappointed that it isn't a player that I rate.
But this is all to do with expectations.
If people 'rate' Darren Bent, Shaun Wright-Phillips, Scott Parker, Stephen Appiah, and Robbie Keane and have decided that they are gettable, then if players who aren't as good as them are signed then they are bound to be disappointed.
At the moment, we don't know whether we will be buying the SWP level of player, the James Milner level player or the Lee Cook level of player. Depending on how you have personally set your level of expectation dictates how disappointed you will be.
There may be fans who have set their level of expectation at the Henry/Gerrard/Terry level of player, and will thus be unhappy if we sign less than that. Who knows.