I was approached by three political campaigns to run as a bound delegate in Maryland this year (long story, but a lot of candidates know and trust me). That's how it's done in some states. In other states, the party selects delegates either before or after the primary election, to represent the interests of the state party, and all active campaigns get as involved as state rules permit in stacking those delegations.
It's not sleaze, but it's not classy either. It's supposed to be to make sure the delegation watches over local interests in the party platform (Ethanol is one such example, and Abortion is another), but it works out in the rare contested convention to be a major factor after the first ballot. I was approached months ago, but I would be startled if Cruz, Kasich, Jeb, Christie (he wants influence!), and yes Trump - that's The Art of the Deal in politics - were not doing this in SC, in LA, and everywhere else, plus lobbying bound delegates to either keep their loyalty or shift their votes after that first ballot.
No doubt Trump would be doing the same with Cruz delagates as you said due to his " deal making".
I don't like it, but your post gives much clarity to the issue.