the better the casino, the less you have to worry about.
basically they don't want people playing deliberately to cash out bonus money without actually "gambling". however, they also get peeved if you whack down most or all of the bonus money in one bet. those are the ideological answers to what they see as bonus abuse; of course standard things like multiple accounts, multiple bonuses, not meeting t&c's, playing disallowed games before wagering is cleared, etc can void your bonus/cashout under "abuse" as well.
your intent shows through; if you play as you would without a bonus, then it's nothing to worry about. if you grind out the exact number of minimum bets to reach the wr, or if you bet huge off the start to make double or nothing the value, then it's obvious you are trying to use the bonus with an intent (this is their favourite line) "contrary to the spirit in which it was given".
to min-grind is to not be gambling but just following instructions toward making bonus chips into real money. to do the quick-double-up is to treat the bonus as worthless to you, ie prepared to lose it in one or two hands. they want you to play as you normally would with your own money (no one throws their entire balance on one roll, well at least not the second they enter the casino), but of course they have to limit how soon you can cash out to avoid the first type who just want the max free cash with the least risk.
lmao i was going to just post the first line of this post, but i figure why not elaborate the reasons behind it as well. but no accredited casino should give you any flak if you're legit, and the best will not even care much to impose the ideological reasons, since technically it's all still betting and not really their business how you choose to bet.
and of course they do still win from the house edge taking its predictable cut (and reserving the right to restrict games and make playthroughs long enough to predictably win back the entirety of the bonus credits paid out), not to mention the big-bettors who get the short end of "double-or-nothing". nonetheless, in their shoes i can see why they view it as harmful/abusive or mean/rude of the player to use the bonuses this way.
as the phenomenon of bonus-wh0ring grew, casinos had to wise up and increase wr's, restrict games, and invent new ways to define improper play. it's just a shame they don't actually specify in t&c which play patterns are considered abusive. still, many explicitly forbid betting for instance red and black together on roulette, but they don't go one more step and say don't bet your whole stack or play to the very dollar of the wagering required...
alas, such is ambiguity... hope that is a decent treatise on bonus abuse. kk, v-dub, anything i omitted? they probably rung in on this while i was typing
edit after seeing above: spot on bryan! spoken like a true player advocate! mine on the other hand, spoken like a crusty cynical player...
edit after seeing below: simmo!! how ya been, old chap? i miss the days when we used to all make fun of you all the time, often in animation as well as just in text. you big blue blinking muppet!