As a freelancer I understand where you're coming from. On the other hand, we're a business. How many times have we gone into a store, and are "just looking"? Went to a lot, test drove a car, and said we needed to leave and "think about it"? Called prospective businesses for pricing and never followed up with them?
Part of doing business is servicing those who may not be paying customers. You must have that time margin built in. Is it rude for potential customers to not follow up, even to let us know nothing's happening? Maybe. I'd never call a lot back to let them know I was just prospecting, either.
If your freelance practice can't bear to lose time like this, you need to rethink your pricing or your chosen profession.
You're thinking about time spent designing/coding as a commodity, which it isn't. A storefront selling widgets exists explicitly to handle a never-ending stream of potential customers, but the investment per customer is very low.
When someone calls and asks for days or weeks of my time to put a quote together (pre-sale & often without pay). And then, if the quote moves forward to billable work, there's an expectation that I'll immediately block off months of my time to hold up my end of the bargain.
I think the relationship is quite different than someone merely "selling something", and should be treated as such.
I think the analogies you make to other types of sales aren't terribly strong. Walking through a brick and mortar store without buying anything takes effectively no time from the business - they can handle many customers in parallel. By contrast, calling prospective business or test driving a car does involve at least acknowledging the seller. You might not tell them you've bought a different car, but you also don't hear the proposed price and hang up the phone without a word, which is a closer analogy to getting an offer and replying with nothing, not even a form letter.
Part of doing business is servicing those who may not be paying customers. You must have that time margin built in. Is it rude for potential customers to not follow up, even to let us know nothing's happening? Maybe. I'd never call a lot back to let them know I was just prospecting, either.
If your freelance practice can't bear to lose time like this, you need to rethink your pricing or your chosen profession.