The ping attribute basically adds click tracking as a native browser feature so you don't need to do URL redirects. It also makes these analytics much easier for the site and mysterious to the user. Looks like most vendors besides Firefox support it. (They were pretty opposed I recall)
If you're a Chrome user, there's some extensions that disable ping requests/link auditing [1]. (EDIT: a commenter noted that uBlock Origin already blocks these! So I recommend that over this obscure extension)
[1] https://chrome.google.com/webstore/detail/ping-blocker/jkpoc...