The NEMA TT-30 outlet is kind of confusing, especially to those with no prior experience to it. It is no excuse for a professional not to know of this variant, or to actually read the outlet/plug itself...but it is easy to get into a groove and not realize it's 120 volt.
Most everyone (including electricians) are going to be in the habit of only seeing 120 volt outlets in the NEMA 5 series. These 5 series always have a vertical straight blade, whether it's the 5-15 amp, 5-20 amp, 5-30 amp, or 5-50 amp even.
Where it gets confusing somewhat, and you could just call it force of habit, is that the NEMA 10 series which is 240 volt has angled blades (very similar to the TT-30 120 volt). If you have never seen or know that the TT-30 is 120 volt, it's an easy mistake to mis-indentify at initial glance, since just about all NEMA configurations with angled blades are 240 or more volts.
Whomever created the TT-30 really could have come up with different blade configuration, but what is done is done now.
This chart will show these NEMA 5, 10, and TT series.
For those reading along, the NEMA 14-50 120/240 volt configuration below is what 50 amp RV's use. This is not a special outlet like a TT-30 is, and is more common to other things. The 50 amp RV utilizes it in it's electrical distribution panel, where although there are actually 2 hot lines feeding the RV, it keeps each line separate and thus 120 volts... just like a 30 amp RV is.