It's complicated from a clubs perspective apparently, its all about tax issues.
If the club organise a testimonial, they have to pay a shitload of tax (if its for charity).
If fans organise a testimonial, with the backing of the club, they get round this tax issue. Something along these lines anyway.
I know some of the fans groups are getting together to try and organise it. Should be interesting if the rumoured opposition is who they have planned.