first of all, you keep saying the teams didn't want to let their players "walk for nothing". You do realize the NFL has compensatory picks right? There is a reason teams are okay to let their free agents leave. Because if its a big free agent who signs a big contract, they will be compensated with a draft pick the following season. That's why teams don't tag and trade their players every year, because most of the time its stupid and pointless.
Cassel was tagged because the Patriots knew they could get more for him by trading him than they would get from receiving a compensatory pick. Since Cassel was quarterback who played well the season prior, the Patriots were able to get a very early 2nd round pick for him. If they would've let him walk, they likely would've received a 3rd round comp pick the next season instead. But since QBs are always in high demand, they were able to get more for him.
With Landry it is different. If the Dolphins would've let Landry walk, next season they very likely would've received a 3rd round compensatory pick for him. Instead, the Dolphins decided to tag and trade him. This is different from the Cassel situation because very few teams, if any, are willing to give up a high draft pick for a WR who they are going to have to immediately pay top dollar for. A WR does not carry a franchise like a QB does. This is why it was stupid that the Dolphins tagged Landry with seemingly only intentions to trade him. They were never going to get more than a 3rd round pick for him. The only reason the Dolphins had any reason to tag and trade Landry is because they didn't want to wait a year to receive their comp pick. But that has proved to be a costly mistake because they are having trouble trading him and they have absolutely no cap to keep him.
Now I assume you struggled to comprehend my post because you were unaware that teams receive compensatory picks, or you wouldn't have said "It's not totally stupid for Miami, they're getting a pick for a player who destined to leave. Even if it's for a 6th round pick at least they get something in return".
Now you probably see why it would be an awful outcome for Miami if they were only to receive a 6th round pick for him. And why it was a stupid idea to tag him with no cap space.
I'm very well aware of compensatory picks, but not every team values them as the Ravens do, and if there is a team that surely doesn't it's the Miami Dolphins which defeats the purpose of bringing the comp pick into the equation in the first place. Teams don't tag and trade players because more often then not, the player they tag is someone they want to keep around in the first place. If they don't keep them around, it's not just because of the compensatory picks, it could literally also be the fact that they can't afford to keep said player or they have a successor in the depth chart. The idea that most teams let players walk due to compensatory picks can be subjective because not everyone values them highly, it's just a nice bonus to have, but nothing worth letting a All-Pro or Pro Bowl player walk out the door for when you can afford to keep him.
As far as Landry getting the Dolphins a 3rd, we don't know that. The NFL has never fully revealed how they give out compensatory picks so it's just left up to guesses in that aspect. Also, Ricky Wagner was supposed to fetch us a 3rd round comp pick, well we ended up with a 6th because of his injury. You can't be sure he's going to get you a 3rd when the formula for handing out compensatory picks is damn near unsystematic. That's also assuming that Miami was just going to sit down and let everyone else spend money in FA when in fact reports are that they aren't.
As far as them getting a 6th in return, yes, you're right, they'd rather let him walk but a 4th or a 5th could get it done because Compensatory picks are handed out in the bottom of the rounds from 3-7. If I'm Miami and I plan to spend money and I know that I might not get a 3rd round pick for him, then I'm fine with accepting a 4th or a 5th.
It's funny how you say that I am unaware of the comp pick formula for saying that they would trade him for anything, when you skipped over all of this.
Also, you said it's stupid for them to tag him when he comes off the books at the start of the league year. Do you really think they're going to manage to be over the cap at the start of the league year? That's literally a prospect that every team, including the Browns manage to avoid, and once the new league year starts, he will be off their cap. I don't see an issue with the cap in regards to the tag when they can easily find a deal for him lol. It literally means nothing lol. In the end, he got traded, and they got two picks for him, that literally beats what they would have gotten if they let him walk.
As far as Cassell goes, I feel like what happened today solves that question. I don't really think the scenario are all that different at all and even if they are, the premise are pretty much identical.