The issue here is whether ATC ever cleared you for the STAR in the first place. There are plenty of times we’ve been close to the first point on the STAR (i.e. our clearance limit for the route) and never received an explicit STAR clearance. Most of the time we query the controller and get an affirmative clearance or a direct or something. (On another note are we are just cleared to fly the STAR if its in the flight plan, without clearance?).
As for assuming if constraints apply, a lot of the time where I fly ATC will explicitly say direct to position XYZ and rejoin the STAR, so we take it to mean that constraints apply unless otherwise instructed or unless we confirm that they don’t. I don’t see how a constraint on a STAR can apply when ATC has given you a direct routing to a point that happens to be on the STAR without any positive clearance to fly the STAR. The fact that waypoint XYZ is on the STAR means nothing if we are going direct and haven’t been cleared on that specific STAR. Otherwise I would expect ATC to inform of us of the requirement to adhere to the constraint either by explicitly stating what speed/alt they want, or by reference to the STAR constraint.