I don't believe that is true. Within html markup ampersands in url parameters must be encoded like ?pageid=x&mid=y but when you click a link in the browser they get decoded like ?pageid=x&mid=y
So if you copy and paste a formatted url with parameters from source view, the encoding will not be correctly decoded therefore the url is not correct.
Cannonical links are not used by end users and not visible to click in the web browser but they must be encoded the same as any other link and search crawlers can click/follow them correctly. If they are not correctly encoded then you will get validation errors from the w3c validator.
My webmaster tools from google show no problems with my cannonical urls on this site.
For example if you look at the forum bread crumb for "Report Bugs" in this page, if you right click it and choose inspect element the browser will show it without encoding of the second url parameter, but if you right click the page and view source you will see it is encoded and clicking it works in the web browser but if you copy and paste the encoded url into the browser navigation it does not work because it wasn't decoded like it would be if clicking the html link.
Hope that helps,