why is [Foucault] the target of a visceral rejection or at least a complete indifference for most of the anarchists?
The author suggests two reasons:
Foucault is characterized by a deep pessimism as to the possibility of getting out of power relations or more precisely of organizing them in an emancipative manner
The second reason for the anarchists’ refusal or indifference to Foucault’s analyses stems from a paradox: the great proximity between both and more particularly, as Tomas Ibanez points out, the importance Foucault grants to the reality of power, its ubiquitous, brutal and insidious character, crossing through the most harmless interactions, organizing in series (in the sense that Proudhon gives to this word) and producing structures of domination (Churches, States, Political parties and the very Individuals) with a capacity of illusion and oppression that does not rely firstly on their blinding visibility but on the tight and often imperceptible network of immediate and tiny dominations of which these structures are but the resultant (Proudhon again).
While these two points may be right, one may also ask why in the first place that Foucault ought to appeal to anarchists (or vice versa). The fact that he doesn’t does indeed point to different conceptions of power.