There is some preliminary evidence from an animal study that the myocarditis side effects aren't being caused by the vaccine per se but rather by bad injection technique.
https://www.cdc.gov/coronavirus/2019-ncov/vaccines/safety/my...
If this was merely application error, ought we expect this reaction to be equally probable after the first and second dosages, this isn't what the data suggests.
This is standard procedure in many communities, I wonder why they seem to be pushing back on this. It seems like a good and harmless argument to me.
Some argued it would increase acceptance and reduce stress if aspiration wasn't used. I dislike this line of reasoning but can understand the overall decision.