Solr does have timezone support: it supports UTC ... that may sound like a cop-out answer, but it's true. DateField specifies that it only accepts UTC formated dates, and stores the UTC date in the index, it knows that a date it receives as input is UTC because it ends in "Z"
In the future, DateField might start allowing documents to be indexed with alternate timezone specifiers, and convert to UTC internally before writing to the index; or new options might get added at some point to allow query clients to specify what timezone they are in, and solr could convert all the internal dates to that timezone for them, etc...
...if/when features like those get implemented, they can only work if there is a standardized internal format, and at hte moment the only way DateField can ensure that there is a standardized internal format is if it forces the clients updating the index to only send UTC dates.
If I'm using Solr and want to feed it dates in a particular time zone, or perhaps a local-time of day, and clients expect this, then why should Solr force me to specify a timezone? I find it irritating.
there's nothing to stop you from lying to solr about the timezone. If all of the update/search clinets for your instance are in on the secret that the times are really GMT-0730 even though solr thinks they are UTC, then no one gets hurt.
But if we droped the requirement that date inputs have the "Z" suffix, people would assume they can index stuff like 1995-12-31T23:59:59-07:30 and then be confused when it doesn't work.