We can only guess why they used a 50A receptacle, confusion or convenience (using what you got) are the top candidates in my opinion.
Using a 50A plug on a 30A drier would be a clear violation of the NRTL Listing which includes the installation instructions that certainly include cord rating, but there is a misaligned interpretation of the NEC eguarding the receptacle:
210.21(B)(1) Single Receptacle on an Individual Branch Circuit. A single receptacle installed on an individual branch circuit shall have
an ampere rating not less than the branch circuit rating.
The Code doesn't clearly prohibit a higher rated receptacle on Individual Branch Circuits in this section. 210.23(B)(3) applies a different rule for multi-outlet circuits.
That being said a NEMA 10 receptacle that provides an equipment ground via the "grounded conductor" (the neutral) is receptacle only on allowed on legacy circuits specific to one exception in the NEC:
NEC 250.140 Frames of Ranges and Clothes Dryers. Frames of electric ranges, wall-mounted ovens, counter-mounted cooking units,
clothes dryers, and outlet or junction boxes that are part of the
circuit for these appliances shall be connected to the equipment
grounding conductor in the manner specified by 250.134 or 250.138.
Exception No. 1: For existing branch-circuit installations only where
an equipment grounding conductor is not present in the outlet or
junction box, the frames of electric ranges, wall-mounted ovens,
counter-mounted cooking units, clothes dryers, and outlet or junction
boxes that are part of the circuit for these appliances shall be
permitted to be connected to the grounded circuit conductor if all the
following conditions are met.
(1) The supply circuit is 120/240-volt, single-phase, 3-wire; or
208Y/120-volt derived from a 3-phase, 4-wire, wye-connected system.
(2) The grounded conductor is not smaller than 10 AWG copper or 8 AWG
(3) The grounded conductor is insulated or the grounded conductor is
uninsulated and part or a Type SE service-entrance cable and the
branch circuit originates at the service;
(4) Grounding contacts of receptacles furnished as part of the
equipment are bonded to the equipment.
Notice the exception applies to existing installations only. For new installations a NEMA 6 must can be used for a grounded 240v receptacle, or a NEMA 14 must be used for a 240/120v circuit that also requires a neutral.