Some people claim the answer is "always" or "never" although this approach seems bizarre when presented with one of the obvious extremes. For instance:
Example #1. You visit a small book shop. You ponder what you might like, and a clerk gives you some excellent advice. You thank them, look at the price, and go and buy it somewhere else. I think most people agree you should avoid doing that -- certainly not set out to do it.
I think it doesn't matter whether the shop has a sign saying "please don't do this" and doesn't matter that if they did it would be unenforceable. I think the shop is implicitly providing a service in the hope that you will be grateful. (I think it's fine to take the advice even if you're only browsing, especially if you make that clear, so long as if you suddenly decide you do do it there.)
Example #2. Conversely, I think Ryanair's business plan isn't just to sell flights at supposedly and deceptively low prices. They do do that, but I think they also hope to make a lot of money out of people who are tricked into thinking it's cheaper, but forgetting that if they want to check a suitcase, or pay by debit or credit card, or forget to pack a sandwich for the flight, they'll pay an exorbitant fee.
However, if the fare is cheaper (which now is often not the case by very much if at all, but sometimes is), I don't feel obliged to follow their expectation and buy lots of overpriced extras. I feel sorry for people who do, but I don't think that if the deal is marginally good for me refusing to participate will help anyone else, so I'll take it, and Ryanair will presumably make their money back from the people who DO buy extras, and if they don't, well, they deserve it.
Further examples: If you're eating in a restaurant, do you feel obligated to buy a drink? If you get a lot of use out of a free version of a program, do you feel compelled to buy the upgrade even if have most of the functionality you need already? If there's a special offer and you assume it's intended to benefit the business somehow, do you mind taking advantage of it?
I think these examples do have something in common, but you should judge whether it's a case of "follow the implied contract" or "Caveat venditor" based on:
(A) Is the vendor setting up an expectation on you, or providing a pricing model and inviting you to partake or not as you choose?
(B) Do you feel the vendor is genuinely helping you and deserves genuine reciprocation, or solely screwing you the maximum amount permissible under law?
Example #1. You visit a small book shop. You ponder what you might like, and a clerk gives you some excellent advice. You thank them, look at the price, and go and buy it somewhere else. I think most people agree you should avoid doing that -- certainly not set out to do it.
I think it doesn't matter whether the shop has a sign saying "please don't do this" and doesn't matter that if they did it would be unenforceable. I think the shop is implicitly providing a service in the hope that you will be grateful. (I think it's fine to take the advice even if you're only browsing, especially if you make that clear, so long as if you suddenly decide you do do it there.)
Example #2. Conversely, I think Ryanair's business plan isn't just to sell flights at supposedly and deceptively low prices. They do do that, but I think they also hope to make a lot of money out of people who are tricked into thinking it's cheaper, but forgetting that if they want to check a suitcase, or pay by debit or credit card, or forget to pack a sandwich for the flight, they'll pay an exorbitant fee.
However, if the fare is cheaper (which now is often not the case by very much if at all, but sometimes is), I don't feel obliged to follow their expectation and buy lots of overpriced extras. I feel sorry for people who do, but I don't think that if the deal is marginally good for me refusing to participate will help anyone else, so I'll take it, and Ryanair will presumably make their money back from the people who DO buy extras, and if they don't, well, they deserve it.
Further examples: If you're eating in a restaurant, do you feel obligated to buy a drink? If you get a lot of use out of a free version of a program, do you feel compelled to buy the upgrade even if have most of the functionality you need already? If there's a special offer and you assume it's intended to benefit the business somehow, do you mind taking advantage of it?
I think these examples do have something in common, but you should judge whether it's a case of "follow the implied contract" or "Caveat venditor" based on:
(A) Is the vendor setting up an expectation on you, or providing a pricing model and inviting you to partake or not as you choose?
(B) Do you feel the vendor is genuinely helping you and deserves genuine reciprocation, or solely screwing you the maximum amount permissible under law?