The dismissal, named after India bowler Vinoo Mankad who ran out Australia's Bill Brown in similar fashion in 1947, is permitted under cricket's laws but viewed by some as going against the spirit of the game.
Buttler departed after an angry exchange with Ashwin and his exit condemned the Royals to a 14-run defeat in their IPL opener in Jaipur.
"On my part, it was very instinctive," Indian off-spinner Ashwin told a post-match news conference. "It wasn't planned or anything like that. It's there within the rules of the game.
"I don't understand where the spirit of the game comes (in). Naturally, if it's there in the rules, it's there."
The Marylebone Cricket Club, the authority on the laws of cricket since it was founded in 1787, interpreted the incident in a technical way without saying whether it thought Buttler should have been judged out or not out.
"Yesterday's incident could have been ruled out or not out, depending on how 'the instant when the bowler would normally have been expected to release the ball' is interpreted," it said.
"If it was a deliberate delay, that would be unfair and against the Spirit of Cricket. Ashwin claims this not to be the case."
That view was not shared by Royals coach Paddy Upton.
"I think we'll leave it up to the IPL fans to decide if that's the kind of things they want to see, and we'll leave it up to the cricket world to judge R Ashwin's actions tonight," Upton said.
"But for us, we're certainly here to play cricket and entertain the fans and be good role models for people who love the game."
Former Australia spinner Shane Warne said Ashwin had committed a "low act".
"So disappointed in (Ashwin) as a captain and as a person," Warne wrote on Twitter. "(Ashwin) had no intention of delivering the ball ... this a not a good look for the IPL ... Why do such a disgraceful and low act like that tonight?"
England limited overs skipper Eoin Morgan said Ashwin had set a bad example for young players, while former England skipper Michael Vaughan suggested the rules should be changed.
"If (Buttler) had been warned, well that's fine," Vaughan Tweeted. "If he hasn't and it's the first time, I think Ashwin is completely out of order ... I would prefer a one warning rule, then after that it's a free for all."
(Reporting by Simon Jennings in Bengaluru; Additional reporting by Hardik Vyas; Editing by Peter Rutherford and Hugh Lawson)
