Egelstaff, a badminton player, says the former world number one has been unfairly accused of acting against the spirit of sport.
Sharapova, the biggest name to test positive for meldonium since it was officially banned by the World Anti-Doping Agency (WADA) in January, admitted using the drug, which boosts aerobic performance, for health reasons.
The Russian, who is provisionally banned pending an International Tennis Federation (ITF) investigation, said she had not read an email saying meldonium had been added to WADA's list of banned substances.
"When Maria Sharapova tested positive for meldonium the backlash was immediate and fierce," Egelstaff told The Mixed Zone website.
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"The outcry focused more on the fact she had been taking meldonium, reportedly prescribed by a doctor for a decade without an apparent medical need for the drug, rather than the fact she failed a doping test.
"The morality police were out in force, decrying Sharapova for taking a drug for its performance-enhancing qualities," added Egelstaff, a Commonwealth Games bronze medallist in 2006.
"This condemnation amazed and frankly stupefied me. There was a remarkable number of people who believed themselves qualified to judge what is morally acceptable in sport."
There have been more than 100 positive tests since then but in some countries meldonium was routinely used as a supplement before it was added to the WADA list.
Egelstaff argues there is no difference between athletes who took meldonium to boost aerobic performance and the use of other legal treatments.
"During qualifying for London 2012 (Olympics) I had severe pain in my foot...I got a cortisone injection and within 24 hours I was completely pain-free," she said.
"This injection was, indisputably, performance-enhancing. Without it I'm not sure I could have continued playing, with it I qualified for Team GB.
"I didn't feel the tiniest pang of guilt about having the injection and neither should I, it was legal after all," added Egelstaff.
On Thursday, Russia's Sports Minister Vitaly Mutko said hundreds of sportspeople in his country used meldonium before it was prohibited.
"Anything that is not banned by WADA is fair game," Egelstaff said.
"WADA has a banned list for a reason, and testing positive for a substance on that list will, quite rightly, result in a ban. There is not another list entitled 'Legal But Morally Wrong'".
(Editing by Tony Jimenez)
