Debbie, that's a really excellent question, and it's one that I've done some research on and have been discussing with Albert and Nick. Here's what I think:
In Iowa towns and other places, a preliminary hearing could and can be a number of different things. Even in Davenport in 1921, a preliminary hearing before a justice of the peace could have different procedural features than a preliminary hearing in a police court.
Bix was brought before Justice of the Peace William W. Scott (not "Judge," despite the police notation), whose office was most likely in the Masonic Temple in Davenport (the older one, not the new one that opened sometime after 1921). In many cases, the preliminary hearing was simply a device for having the nature of the charges explained to the accused; it was not necessarily a setting in which the accused pleaded guilty or not guilty (contrary to certain suggestions on this list a year or two ago).
I'm pretty sure that Bix's preliminary hearing was of the non-pleading type, and this type of hearing, I've found, was often waived by the accused, including by those charged with lewdness. That Bix waived the preliminary hearing almost certainly means that he was represented by counsel. (I'd love to know who.)
After that formality was waived, Justice Scott went to the issue of bail. $1,500 was quite a substantial bail amount for a criminal case but not atypical for charges concerning this particular felony. (I saw numerous ones set at $1,000 or more.) I don't know who paid the bond, whether the Beiderbeckes put all of it up or they obtained the sum from a third-party surety (or bail bonder).
Let me know if that helps clarify this area. Best wishes, Bob