you indicated this is due to some recent change? - was it ever the case in the past that those features of those websites did work with icecat?
those "popular" websites are both proprietary - it is quite likely they are serving non-free javascript, and supporting only "popular" browsers with anti-features "clipboard events", for example, is considered to be an anti-features; because it allows the javascripts to know exactly what the user copied or pasted - the "clipboard events" feature of browsers is not actually required to copy-paste text - if copy-paste does not work on some website without enabling the "clipboard events" feature, that is only because that website insists on it happening only in that way that is perhaps, not the answer to the OP, but for perspective of the big picture, consider this: you are correct that some people may decide not to use a free software web-browser if it interferes with their ability to enjoy some non-free web-sites with anti-features other people may decide to avoid such web-sites that do not work well with their free software, privacy respecting, web-browsers even if this were an unintended behavior of icecat, in the "big picture", there is really no flaw described here - it is just a matter of opinion, and GNU would/should prefer to encourage people to adopt the latter opinion -- http://gnuzilla.gnu.org
