Is the max pressure on tubeless rims specifically for tubeless, or do they apply to tubed tires too?
As for rim strong enough for highest.  The force the rim must take is both psi and size.  If you have twice as many molecules pushing 1/2 as hard the total force is the same.  It is not exactly force on the rim bead but the force on the tire to pull it off the bead.

Here is a reference where they give different pressure for tube versus tubeless. 
Hutchinson tires information  See question 7. 
I just bought some Pyranha 2 CX tubeless and the pressure on the box is different from the pressure on the tire.  Box is clearly the tubeless pressure as it is in the tubeless mounting instructions.

It is a lot about how the and where tire engages rim. 
On Stans (ZTR) the bead is shallow so you get more tire evolved in the adsorb rocks side. 
On Stans I would assume they mean max pressure period until you see a different pressure listed for with tubes.  Their story is with a more shallow rim you need less pressure. See the diagram in this literature that standard rim with tube engages differently.
Comparison of a standard bicycle rim to a Stan’s BST Rim  

How the different rims do UST varies.  And how they cross to tube varies.  Some have more of a dual type of engagement and some are designed just for UST. 

ztr_rim_specifications 


  Stan’s rims are a tubeless design and care must be taken when using
  tubes.


If you want to go both ways then don't go Stan's.

Stans has great support - you should try and contact them.

See the two diagrams below.  The first is a Stans and the second a Mavic. 
In these examples the Mavic has a separate groove for UST - a tube tire will be on the outer part of the rim. 
According to DWGKNZ the Mavic will seat on the outer even tubeless. 
Still you have different designs and some are designed for tubeless only and some fro both ways. 



I am learning from comments from DWNG.
UST is a tighter standard than I was aware. The bead must have a specific design.
UST tubeless-tire system
There are still variations in the implementation. 
Some rims will cross over better than others.