Here's the dress code from NBA.Com.
1. General Policy: Business Casual
Players are required to wear Business Casual attire whenever they are engaged in team or league business.
"Business Casual" attire means
A long or short-sleeved dress shirt (collared or turtleneck), and/or a sweater.
Dress slacks, khaki pants, or dress jeans.
Appropriate shoes and socks, including dress shoes, dress boots, or other presentable shoes, but not including sneakers, sandals, flip-flops, or work boots.
2. Exceptions to Business Casual
There are the following exceptions to the general policy of Business Casual attire:
a. Players In Attendance At Games But Not In Uniform
Players who are in attendance at games but not in uniform are required to wear the following additional items when seated on the bench or in the stands during the game:
Dress shoes or boots, and socks
b. Players Leaving the Arena
Players leaving the arena may wear either Business Casual attire or neat warm-up suits issued by their teams.
c. Special Events or Appearances
Teams can make exceptions to the Business Casual policy for special events or player appearances where other attire is appropriate -- e.g., participation in a basketball clinic.
3. Excluded Items
The following is a list of items that players are not allowed to wear at any time while on team or league business:
T-shirts, jerseys, or sports apparel (unless appropriate for the event (e.g., a basketball clinic), team-identified, and approved by the team)
Headgear of any kind while a player is sitting on the bench or in the stands at a game, during media interviews, or during a team or league event or appearance (unless appropriate for the event or appearance, team-identified, and approved by the team)
Chains, pendants, or medallions worn over the player's clothes
Sunglasses while indoors
Headphones (other than on the team bus or plane, or in the team locker room)
Nothing about tucking a shirt in.
Sounds quite reasonable