The striker was dismissed from the camp on Monday along with Stoke’s Rabbi Matondo and Tyler Roberts of Leeds, a player who came through the youth ranks at The Hawthorns.
And Allardyce revealed Robson-Kanu’s misjudgement was related to coronavirus restrictions.
He said: “It’s one of those things where you just slightly break what you should be doing by meeting somebody you shouldn’t and they decided it was enough to send him back.
“He’s been tested and he’s obviously got a negative test and he was back in training today (Thursday).
“We can all make an error. It depends on the size of the error.
“I think on listening to what he said it was minimal to say the least.”
Robson-Kanu is now likely to be in Albion’s squad for Saturday’s Premier League clash with Chelsea but young midfielder Conor Gallagher will be missing under the terms of his loan agreement from the London club.
The 21-year-old has impressed, starting 22 league games this season, leading to speculation he could force his way into the first-team picture at Stamford Bridge next term.
But Allardyce believes Gallagher would benefit most from another spell at The Hawthorns next season – if they stay in the Premier League.
“If we stay in the Premier League, I’d hope he’d be here next season not at Chelsea,” the boss said.
“I think he’d be better off having another season with us rather than the competition and size of Chelsea’s squad.
“I think his learning and development level has been improved.
“I’ve seen a good player since I’ve been here.
“I heard about how good he was before I came and think every day and every game he plays in the Premier League he gains more and more experience.
“When you lose more than you win, which he’s experiencing for the first time, it makes you tougher, it makes you more determined.
“And at such a young age to get these disappointments week in and week out could only stand him in good stead about improving his career.”