There is a clear difference between a company deciding how it sells shares it currently owns in an IPO and restricting someone from buying shares from other parties.
The article isn't entirely clear though, at first glance it makes it sound like Icahn is being frozen out from buying the company, however I think his offer as far as that goes is still on the table. This agreement seems to be more about preventing a
hostile takeover, ie Ichan gaining enough shares/influence in the meantime to control the process and have his offer approved with less oversight.
As it's an agreement Ichan is presumably OK with it though if you were a Dell shareholder you might be annoyed that the company has taken someone who might buy your shares out of the market for a while.