ASCII bit trick to convert lowercase to uppercase and back

For a fun application, this trick is used in a proposal to encode extra randomness into the 0x20 bit of each character of a DNS query (to increase resistence to poisoning attacks):

https://tools.ietf.org/html/draft-vixie-dnsext-dns0x20-00

Is it a "trick" if the designers of the original ASCII chart obviously designed it like this?

Yes ASCII (and UTF-8) were designed well.

http://www.pixelbeat.org/docs/utf8_programming.html