Where do you find that data? Regardless, the overall lower 48 anomaly is the most even way at looking at it across the country.
Certainly not comparable to Mexico's May heat dome, which broke tons of all-time heat records for many cities including Mexico City and was also responsible for the hottest temperature on record for the country.
There wasn't that kind of extreme heat in the U.S. last month, and thankfully doesn't look to be so far in July.
Here in Denver, part of the region that had some of the warmest anomalies last month, there was a lot of moderate heat but nothing truly high end. One day that hit 100...June 2021 had 101, 100, 100; June 2018 had 101, 105; June 2012 had five straight days of 102+, including two 105s; June 1994 hit 104; June 1954 hit 102). There were just hardly any below normal days this year.
I'm sure most fans don't live in the core of the city, events like that typically draw families from all over the region. The Mariners have tons of fans in Idaho, Montana, rural Oregon, rural Washington, and Alaska due to their status as a regional team.
And lets be real, the demographics that like baseball are going to be a demographic that boos fireworks or hates country music.