The striker was dismissed from the camp on Monday along with Stoke's Rabbi Matondo and Tyler Roberts of Leeds, and Allardyce revealed Robson-Kanu's misjudgement was related to coronavirus restrictions.
The Baggies boss 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. 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."
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, saying: "If we stay in the Premier League, I'd hope he'd be here next season not at Chelsea.
"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."
Speculation about Allardyce's position continues, with the Baggies 10 points adrift of safety with only nine games remaining ahead of Saturday's trip to Chelsea.
Allardyce signed an 18-month deal when he replaced Slaven Bilic in December but he insisted the international break had not been an opportunity for thinking about next season.
"No not me personally because it's about us trying to stay in the Premier League and trying to pull off what's been done before," said Allardyce, who was bullish about his team's improvement.
"Many managers have managed to do it, I've managed to do it at other clubs. Recently it was probably Sunderland and Crystal Palace, where I've won six out of eight, or five out of six, so it can be done if the players want to have a real go.
"We must start taking chances, because our chance ratio and our creation ratio is massively improved. Our final-third entries have gone up massively in the last 10 games or so.
"People have said outside, 'Oh they don't create enough chances'. The answer to that is they are absolutely totally and utterly wrong. Yes we do but we don't hit the target and we don't score."