The 25-year-old flanker, who led the British and Irish Lions to victory over Australia last year, became the first player to sign up with the Wales Rugby Union (WRU) over the weekend.
Warburton's Cardiff Blues contract is due to expire later this year.
The central contract with the WRU, seen as a way to stop Welsh talent moving overseas, will mean Warburton can be loaned back to the Blues at no cost to the club when he is not needed for international duty, provided the WRU secures an agreement with the four domestic regions.
Some supporters appeared confused by the deal and called him a “scab” in the belief that Warburton had refused to renew his contract with Cardiff.
But his agent explained that the WRU deal was the only way to keep the player in Wales, as his team were unable to offer him a UK market value contract due to ongoing political problems between the national union and the regions. These issues saw Leigh Halfpenny leave Wales for Toulon this month, and could have seen Warburton double his money by joining that club or Stade Francais.
"Some of the uninformed vitriol aimed at my client has been disgraceful," Warburton's agent Derwyn Jones told The Times.
"I cannot comment on many aspects of Sam's contract negotiation process due to client confidentiality but the broad facts are as follows: Cardiff Blues and the WRU could not table a joint offer for Sam because of the current political fallout between the Welsh regions and the WRU.
"Cardiff Blues alone could not give Sam a market value contract offer due to the uncertain future of the game in Wales. They also could not offer this to Leigh Halfpenny who has since signed for Toulon.
"Sam was faced with the following two options regarding his playing future: pursue interest from two teams in the French Top 14 at twice his UK market value (or) sign a three-year employment contract with the WRU to stay in Wales at his UK market value."
Most of the abuse appears to have since been deleted from Twitter.
"At this time, however, due to the current fallout between the regions and the WRU, there are no cast iron guarantees that Sam will be able to play for Cardiff Blues next season," said Jones.
"Sam wants to stay in Wales and intends to play for Cardiff Blues next season should all the issues be resolved. But if he had moved to France there would have been absolutely no chance of him playing for the team he has supported since he was a young boy.
"He also prefers staying in Wales over moving to France as he is passionate about maintaining the Wales captaincy. Sam appreciates that playing for Wales is a privileged position and does not, in any way, take this for granted."
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