I mean, 95% or more of the blame here is on the engaged couple. How do you get far enough into the wedding planning process that you'd lose $5k worth of deposits without figuring out the small issue of paying for a $30k wedding?
When I was applying to colleges, my parents never really sat down with me and discussed how much they could contribute. They had a vague line of "if you really want to go to X college, we will make it work". I found out during my freshman year of college that "making it work" involved me taking on quite a bit of loan debt and I started to panic about how I would pay it back. I was also miserable at my dream (expensive) college so transferred to a cheaper state school. Somewhere during my sophomore year my dad suddenly told me that he felt a reasonable contribution from him each year was $5k. Which is certainly generous, but is no way going to make a $30-35k/yr college "work". So basically, my parents encouraged me to apply to all these expensive colleges with no real idea how to pay for it, reassured me that things would "work", then kind of flaked (relative to their original assurances). Ultimately I finished college with about $15k in debt that I would not have had if I had gone straight to the state school.
When my husband and I decided to get married, we eloped at the courthouse with no family because I couldn't deal with the idea of planning a wedding with my parents getting involved. We briefly promised to have a party/reception afterwards and I had to quickly put the kibosh on it because guess what, my parents were talking about me needing to do all sorts of things that were going to drive up the cost, assuring me that "we'll help pay for it, don't worry about money" and being incredibly vague about what exactly they would cover or what their budget was.
Ultimately we had a small religious ceremony a few months later with just my immediate family. My husband and I paid for all the things that were important to me (namely, a donation to the church and getting a nice studio portrait of ourselves afterwards). My parents paid for some massive floral arrangement even after I explicitly told my mom I didn't want the flowers and found them kind of sad/wasteful ever since I had worked in the back of a grocery store and spent a couple of weeks over Christmas unpacking flowers that had been flown in from South America. My parents also then insisted on paying for a big dinner out at a restaurant of their choice, which was quite nice but which I also did not care that much about. So basically, my parents didn't particularly listen to me and didn't really help out with the expenses related to anything that was important to ME. Mom actually also freaked out as we were walking into the church about whether or not I had made a donation, telling me that "You HAVE do to that" and implying that I had either forgotten or was too cheap to make a donation. So ultimately, I actually wound up paying for something that was important to HER.
All of that is to say, if your parents are like my parents, I can see how you can get quite far into wedding planning without really knowing who is paying for what and while also having a strong impression that your parents are going to kick in a lot more than they eventually do.
In fairness to my parents, I was the first child to attend college and also the first to get married, so they didn't have a lot of experience of what the actual costs might be themselves, which I suspect partially explains their behavior. Still, I am the one who got burned.