It is but there are a lot of reasons why its probably right that it does go ahead fully attended.
First, as you say, it's a day before the restrictions are going to be fully lifted. Purely from a pragmatic and financial point of view it would have been incredibly harsh to have it behind closed doors and then a few hours later have everything fully open.
Secondly, although 140000 people does sound a lot, they are spread out over the area of a decent sized town when placed around the track, and an even wider area when you include the campsites that surround it.
Third, the whole event is probably 99% outdoors for spectators. Most of the grandstands are uncovered and even the ones that aren't are scaffolding type structures (think the Gene Kelly stand) with no areas underneath that people can congregate or mix like you get a normal sports stadiums.
Fourth, and this is perhaps the most important, I'm going so don't want to miss out again having rolled my tickets over from last year!