Have just had an email from IDG asking to complete short survey and go in draw to win £100 Amazon vouchers. One question asked for income details and needed a compulsory answer. I declined so will not be in the draw !
but I doubt that they included a request for "income details" - you were more likely to have been asked to select from a list of bracketed figures.
There's nothing sinister about it - lots of surveys ask for the information because it helps people like us to offer readership profiles to potential advertisers. There would be no point in trying to attract ads from Rolls Royce if most of our readers were earning less than £10,000 a year.
The information is not disclosed to anyone at all; we use it to compile statistics - 'X% of our readers are between the ages of A and C, and are earning between £D and £F per annum'
It's obviously entirely a matter for you to decide whether or not you provide income information, and you decided not to.